Tore Slagsvold

• Slagsvold, Tore & Husby, Magne (2026). Birds Decorating Their Nests With Plastic May Suffer Less Egg Depredation by Corvids. Ecology and Evolution. 16(1), p. 1–10. doi: 10.1002/ece3.72966. Full text in Research Archive Show summary

  Many birds add anthropogenic material to the nest. This may increase the probability of total failure because the nest may be more easily located by enemies. However, the material may also induce a threat response in predators sceptical to new objects (the Neophobia Hypothesis). We presented artificial nests on the ground each with two quail eggs, in territories of Eurasian mag-pies Pica pica in spring. Some nests were decorated with pieces of white plastic while others were not (control). When nests of both types were presented simultaneously on a magpie territory and only a meter apart, depredation started later for nests with plastic than for control nests, supporting the Neophobia Hypothesis. When a trial was repeated on the same territory later in the season, predation started sooner. However, this was probably caused by habituation to the experimental set up (wildlife camera and artificial nests) and not to the plastic itself because in the repeated trials, the eggs in the nests with plastic were still depredated later than the eggs in the control nests. The nests were not depredated sooner if similar experiments had been conducted on the same territory in the previous year. The onset of depredation was no sooner in territories that initially contained plastic close to the magpie nest than in territories containing no plastic. Finally, when only a single nest was presented on a magpie territory, the time lag until depredation was similar for decorated and control nests, suggesting that the increased detectability caused by decoration outweighed the fear response to the plastic. We conclude that the Neophobia Hypothesis may be relevant to natural cases including birds nesting in habitats containing anthropogenic material and to circumstances with repeated visits by corvids to bird nests, such as in a bird colony.

• Dunker, Henning & Slagsvold, Tore (2025). Secretive flight behaviour of Golden Eagles (Aquila chrysaetos) in the vicinity of their nest. Ornis Norvegica. 48, p. 7–11. doi: 10.15845/on.v48.4420. Full text in Research Archive Show summary

  During breeding, most birds have to visit and leave the nest regularly to forage or to provision young. Nesting activity may help competitors or predators to locate the nest and offspring, selecting for parents to behave secretively. The present study reports observations of secretive flight behaviour of Golden Eagles Aquila chrysaetos during a long-term study in Norway based on more than 1500 hours of observation in the nesting areas of 16 pairs of eagles. By a combination of absence from the nest and secretive flights, the eagles seemed to avoid being observed near their nest, as shown by observations of 14 cases of secretive flights from 10 nesting areas. Silhouettes against the sky near the nest seemed to be avoided by the eagles, either by downhill flight in the forest below the nest and skyline, or by a fast approach to the nest, the latter either by a sideway descent or by falling like a stone from a high altitude above the nest within a few seconds. Secretive behaviour may help to avoid long-distance discovery and nest predation, in particular by intruding non-territorial eagles, but also by other raptors and Ravens Corvus corax, as well as to avoid disturbance by humans. Secretive behaviour creates serious challenges for monitoring of Golden Eagle populations, where each pair of eagles usually have several nest sites and where often less than 50% of the pairs are breeding in a given year.

• Slagsvold, Tore & Wiebe, Karen (2025). Diminishing avoidance over time of nest cavities containing feathers by passerine birds. Behaviour. ISSN 0005-7959. 162, p. 1061–1077. doi: 10.1163/1568539x-bja10337. Full text in Research Archive Show summary

  Abstract Secondary cavity nesting birds often investigate unfamiliar, dark holes while prospecting for a nest site. Such cavities may be dangerous to enter if there is a concealed predator or other occupant inside or if a predator returns to the cavity. Previous experiments on passerine birds showed that fear of entering unexplored cavities by prospecting birds was triggered by a few light-coloured feathers in the cavity which may act as cues of predation. Decorating the nest with feathers may thus reduce the risk of nest usurpation and egg dumping. We tested whether this fear of feathers also deters breeding by letting prospecting pairs of blue tits Cyanistes caeruleus and pied flycatchers Ficedula hypoleuca choose between a dyad of nest boxes which were similar in size and interior colour, but where only one nest box contained three white hen feathers and the other nest box served as a control. As many as 40% ( ) of the pairs chose the box with feathers, suggesting that most of the fear of feathers diminishes soon after the cavity is first entered. Features of the nest box (size and internal colour) did not seem to affect the strength of avoidance of the feathers. The degree of avoidance was similar in the two species, and most pairs of both species removed feathers from the nest box if they eventually used it for nesting.

• Lifjeld, Jan Terje; Kleven, Oddmund; Amundsen, Trond & Slagsvold, Tore (2025). Extrapair copulations in coal tits (Periparus ater): female insurance against male infertility? Behavioral Ecology and Sociobiology. ISSN 0340-5443. 79. doi: 10.1007/s00265-025-03621-4. Full text in Research Archive Show summary

  The evolutionary causes of female extrapair mating in socially monogamous birds are poorly understood. Adaptive hypotheses focus on direct benefits, i.e. fertility and paternal care, or indirect, genetic benefits, but the empirical support is generally sparse and mixed. Here we report a study of extrapair paternity in a Norwegian population of coal tits (Periparus ater), a species in which previous studies have reported a high frequency of extrapair paternity, but with no clear support for any female benefits. We confirmed a high occurrence of extrapair paternity (32% of offspring) and found that most extrapair sires were breeding neighbours. Extrapair offspring were heavier than their withinpair nestmates during the early nestling period, indicating that they were among the earlier eggs in the laying sequence. We evaluate various hypotheses for female extrapair mating and conclude that a direct benefit in the form of insurance against mate infertility is a plausible explanation to our results. There is empirical evidence for a low, but presumably biologically significant rate of infertile males in several passerine species, including other tit species. Fertility insurance may have a wider relevance as a direct benefit of female extrapair copulation than previously acknowledged, and male infertility warrants more scrutiny in wild birds in general. Significance statement Nearly one third of all offspring in coal tit broods were sired by a male neighbour. Data on early nestling body mass hierarchy indicate a paternity skew in the egg-laying order. The results are consistent with the fertility insurance hypothesis, which states that female birds engage in extrapair copulations before the start of egg laying to safeguard against a potentially infertile mate. However, direct evidence for male infertility is lacking and further studies are needed.

• Husby, Magne & Slagsvold, Tore (2025). The Neophobia Hypothesis: nest decoration in birds may reduce predation by corvids. Royal Society Open Science. 12(4). doi: 10.1098/rsos.250427. Full text in Research Archive
• Slagsvold, Tore (2024). Intrusion of birds into nest cavities depends on the presence of feathers in the cavity and not on species of nest owner. Ethology. ISSN 0179-1613. doi: 10.1111/eth.13501. Full text in Research Archive Show summary

  Most animals live and move in a landscape in which the risk of predation varies spatially and temporally, that is, a “landscape of fear” which may strongly affect their ecology and behaviour. Cavity-nesting birds prospecting for nest sites are often forced to investigate unfamiliar, dark holes that may be dangerous to enter because they may conceal an aggressive nesting bird or a predator. The Fear of Feathers Hypothesis posits that some birds such as blue tits Cyanistes caeruleus, add large, conspicuous feathers on top of their own nest to exploit the fear of potential nest usurpers or brood parasites. Feathers may trigger fear in prospecting birds either because the feathers obstruct the view of the cavity interior, and/or because the feathers may be perceived as recent prey remains and suggest the predator will return. Here I studied the prospecting behaviour of unmated male pied flycatchers Ficedula hypoleuca by letting them choose between a dyad of nest boxes where one contained a nest of a great tit Parus major and one a nest of a blue tit. The objective was to test whether the presence of feathers would have a stronger repelling effect than the species that had built the nest. Great tits are larger than blue tits and may represent a greater threat to intruding flycatchers. However, blue tits but not great tits, often decorate their nest with large, conspicuous feathers. Consistent with the Fear of Feathers Hypothesis, flycatchers hesitated longer to enter blue tit than great tit nests but only if the blue tit nest contained feathers. The study has relevance to many species of birds that decorate their nest with feathers. It shows that a landscape may include aspects of fear that are important to animal behaviour although they may seem subtle and are easily overlooked.

• Slagsvold, Tore & Wiebe, Karen (2024). Birds do not use social learning of landmarks to locate favorable nest sites. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 78(71). doi: 10.1007/s00265-024-03485-0. Full text in Research Archive Show summary

  Experiments since the classic studies of Niko Tinbergen have provided evidence that animals use visual landmarks to navigate. We tested whether birds use visual landmarks to relocate their nest sites by presenting two species of cavity nesting birds with a dyad of nest boxes with different white markings around the entrance, a circle or a triangle. When the two boxes were erected in close proximity on the same tree, pied flycatchers Ficedula hypoleuca and blue tits Cyanistes caeruleus confused the entrance of their nest both when the boxes had different external markings and when they were unmarked. Most females added nest material to both boxes of a dyad and one third of the flycatchers laid eggs in both boxes although a female can only incubate the eggs in one nest at a time. Thus, the birds did not use external markings around cavity entrances for orientation. We also tried to replicate a previous study purporting to show that migratory birds use social learning of the external appearance of nests from other species. However, pied flycatchers did not choose boxes with the same painted markings as those applied to nests of resident great tits Parus major which were judged to be high quality “demonstrators” from their large clutch sizes. We argue that conclusions from previous studies on social learning based on external markings as landmarks on nest cavities in birds need to be reconsidered.

• Spurgin, Lewis G.; Bosse, Mirte; Adriaensen, Frank; Albayrak, Tamer; Barboutis, Christos & Belda, Eduardo [Show all 46 contributors for this article] (2024). The great tit HapMap project: A continental-scale analysis of genomic variation in a songbird. Molecular Ecology Resources. ISSN 1755-098X. 24(5). doi: 10.1111/1755-0998.13969. Full text in Research Archive Show summary

  A major aim of evolutionary biology is to understand why patterns of genomic diversity vary within taxa and space. Large-scale genomic studies of widespread species are useful for studying how environment and demography shape patterns of genomic divergence. Here, we describe one of the most geographically comprehensive surveys of genomic variation in a wild vertebrate to date; the great tit (Parus major) HapMap project. We screened ca 500,000 SNP markers across 647 individuals from 29 populations, spanning ~30 degrees of latitude and 40 degrees of longitude – almost the entire geographical range of the European subspecies. Genome-wide variation was consistent with a recent colonisation across Europe from a South-East European refugium, with bottlenecks and reduced genetic diversity in island populations. Differentiation across the genome was highly heterogeneous, with clear ‘islands of differentiation’, even among populations with very low levels of genome-wide differentiation. Low local recombination rates were a strong predictor of high local genomic differentiation (FST), especially in island and peripheral mainland populations, suggesting that the interplay between genetic drift and recombination causes highly heterogeneous differentiation landscapes. We also detected genomic outlier regions that were confined to one or more peripheral great tit populations, probably as a result of recent directional selection at the species' range edges. Haplotype-based measures of selection were related to recombination rate, albeit less strongly, and highlighted population-specific sweeps that likely resulted from positive selection. Our study highlights how comprehensive screens of genomic variation in wild organisms can provide unique insights into spatio-temporal evolutionary dynamics. adaptation, birds, ecological genetics, genomics/proteomics, molecular evolution, population genetics – empirical

• Rydgren, Knut; Indreeide, Bendik; Slagsvold, Tore & Lampe, Helene M. (2023). Nest building in titmice Paridae: Selectivity in bryophyte use. Ecology and Evolution. 13(3). doi: 10.1002/ece3.9852. Full text in Research Archive Show summary

  In many bird species, reproductive success is dependent on nest quality. However, detailed data on nest composition are scarce, and quantitative analyses have generally used only rough categories, without species identification. Bryophytes dominate the nests of many passerine bird species, but little is known about whether birds have preferences for certain species. In this study, we determined the bryophyte species composition in nests of blue tits Cyanistes caeruleus and great tits Parus major in a forest near Oslo, Norway. We also sampled the abundance of the bryophyte species in plots on the forest floor surrounding a subset of the great tit nests. Blue tits and great tits both used 15 bryophyte species as nest materials, mainly the same pleurocarpous species but in different proportions. The tits preferred highly branched bryophyte species, i.e., Pleurozium schreberi, Rhytidiadelphus squarrosus, and Sanionia uncinata but avoided common forest floor bryophyte species that are sparsely branched. Great tits clearly collected bryophyte species selectively. We also found that bryophyte species content in great tit nests in the same nest box in different years was very similar. Our results also indicated that the great tits collected bryophyte nest materials close to their nests, mostly within 5 m, supporting the view that collecting nest materials is costly. We review several hypotheses to explain why the tits prefer certain species of bryophytes as nest materials. These include handling costs and their suitability as structural materials. We recommend field experiments to test specific hypotheses and to study whether preferences are heritable.

• Husby, Magne & Slagsvold, Tore (2023). The number of syllables per phrase in songs of Great Tits Parus major decreases with high levels of anthropogenic noise and at northern latitudes. Ornis Norvegica. 46, p. 28–42. doi: 10.15845/on.v46.3854. Full text in Research Archive Show summary

  Birds often sing to defend a territory and to attract a mate. However, despite many studies, clear questions remain on how ecological conditions may affect the song, such as physical obstacles that may reduce the sound transmission, and anthropogenic noise that may mask the signal. The social environment of the local populations may also be important, such as breeding density and sex ratio, influencing the number of competing males with which to song match, and distances to neighbouring males and to prospecting females. During 2016-19, we counted the number of syllables (notes, elements) per phrase of singing male Great Tits Parus major by visiting seven countries in Europe and one country in North Africa. A total of 762 songs were observed by visiting 590 sampling points. We also recorded study year, levels of anthropogenic noise, calendar date, time of day, type of habitat and vegetation density, latitude, longitude and altitude. The most important explanatory variables for variation in songs were anthropogenic noise and the latitude of the sampling points; the number of syllables per phrase decreasing both with increasing levels of anthropogenic noise and with latitude. The latter result was also supported when listening to and analysing sonograms of Great Tits available at the Xeno-canto repository for sound recordings, with fewer syllables per phrase for birds in Norway than in Spain and Portugal. We suggest that repetition of a short phrase is quickly interpreted by conspecifics in noisy environments, and that such signals are more readily detected by conspecifics over a wider area where the density of the birds is low.

• Walseng, Bjørn; Durant, Joël; Hessen, Dag Olav; Jerstad, Kurt; Nilsson, Anna & Røstad, Ole Wiggo [Show all 7 contributors for this article] (2022). Long-term annual and spatial variation of polygyny in the white-throated dipper (Cinclus cinclus). Ecology and Evolution. 12(10). doi: 10.1002/ece3.9416. Full text in Research Archive Show summary

  Mating strategies are key components in the fitness of organisms, and notably in birds the occurrence of monogamy versus polygyny has attracted wide interest. We address this by a very comprehensive dataset (2899 breeding events spanning the years 1978–2019) of the white-throated dipper Cinclus cinclus. Though the mating system of this species has been regarded as generally monogamous, we find that 7% of all breeding events were performed by polygynous males (approximately 15% of all pairs). The fraction of polygyny has been stable over the entire study period irrespective of population size. The assumption that polygyny is most common at low population density was not supported. Surprisingly, there was no difference between polygynous and monogamous males with regard to the quality of the territories they inhabited, ranked according to their overall frequency of use. The most common age group, first-year breeders, dominated among monogamous males, while among polygynous males second-year breeders were most common, followed by third and first-year breeders. The primary females were in general older than females mated to monogamous males, also when controlled for their general frequency in the population. The majority of the two females mated to a polygynous male, bred in the vicinity of each other. The probability for a male to be involved in polygyny more than once, was significantly higher than by chance, suggesting phenotypic quality differences among males.

• Engebretsen, Stein Ingolf & Slagsvold, Tore (2022). An experimental study of trade-offs between brood size and moult in the blue tit (Cyanistes caeruleus). IBIS. ISSN 0019-1019. doi: 10.1111/ibi.13172. Full text in Research Archive Show summary

  Life-history theory predicts that whenever expensive activities overlap, there will be a trade-off between the activities. In a field study of Blue Tits Cyanistes caeruleus in Norway, we tested whether parental energy expenditure during reproduction affected timing of moult of primary feathers. Five days after hatching, we manipulated broods to contain either two (n = 10 broods) or 9–12 nestlings (n = 13). In contrast to previous studies, we measured stage of moult at two time points during the nestling period (7 and 14 days after hatching). The onset of moult in the male but not in the female was strongly affected by a reduction in brood size. Stage of moult did not differ between the experimental groups 7 days after hatching, but did after a further week. This difference was also evident when considering only males that had started moulting on day 7. Male moult score was not related to male age (yearling or older), body size (wing length, tarsus length) or hatching time (calendar date) of the brood. However, body condition, expressed as residuals of the regression of body mass on wing length, tended to be greater for parents of both sexes attending small broods than large broods. None of 23 females had initiated moult 7 days after hatching, and only two had done so by day 14. Surprisingly, moult did not seem to affect food provisioning by males relative to females. These results confirm previous studies of passerine birds, namely, that males may start moulting before females and that the moult can quickly be adjusted to the demands of brood-rearing. The novel result in this case was that such demands affected not only the timing of moult onset but also its speed once it had started.

• Slagsvold, Tore & Wiebe, Karen (2021). No evidence that nest site choice in Pied Flycatchers is mediated by assessing the clutch size of a heterospecific, the Great Tit. Journal of Ornithology = Journal fur Ornithologie. ISSN 0021-8375. 162, p. 997–1007. doi: 10.1007/s10336-021-01900-1. Full text in Research Archive Show summary

  Abstract Among species that use similar resources, an individual may benefit by observing and copying the behavioural decision of a heterospecific. We tested the hypothesis of heterospecific social learning in passerine birds, namely that a migrant species, the Pied Flycatcher Ficedula hypoleuca, uses external markings on the nest cavities of a resident species, the Great Tit Parus major , as cues when choosing a nest site. Others have suggested that prospecting flycatchers assess the clutch size of tit “demonstrators” by entering their nest boxes and, assuming that a large clutch indicates a high-quality individual, will copy the nest appearance of tits with large, but not small clutches. During a 4-year period in Norway, we designed a similar study but did not find that flycatchers based their nest choice on the clutch size of tits. Neither were there any relationships between clutch size of the tit and its laying date, incubation behaviour, or the number of eggs visible through nest material during egg-laying so Pied Flycatchers did not use these indirect cues to assess quality of the tutor. Filming of tit nests showed that prospecting flycatchers did not enter tit nest boxes to assess the content. Indeed, incubating female tits only left their nest boxes for short bouts of unpredictable duration so there was little opportunity for flycatchers to inspect the nest contents unnoticed. Our study calls into question the mechanism of using the content of tit nests as public information for choosing traits of nest sites based on external characteristics. We suggest that similar studies of nest site choice in relation to possible social information transfer be replicated more widely.

• Slagsvold, Tore & Wiebe, Karen (2021). Egg covering in cavity nesting birds may prevent nest usurpation by other species. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 75. doi: 10.1007/s00265-021-03045-w. Full text in Research Archive Show summary

  Abstract Some birds cover their eggs with nest material when they leave to forage. It has been suggested that such egg-covering aids thermoregulation or prevents predation but here we present a new hypothesis, that secondary cavity-nesting species cover their eggs to prevent nest usurpation by other birds. When the bottom of the cavity is dark, as when eggs are covered by nest material, it may be difficult for a prospecting competitor to see whether a defending nest owner or a predator is hiding inside the cavity. Competitors may therefore hesitate to enter dark cavities. We filmed 21 great tit ( Parus major ) nests during the egg-laying period and found that the female spent bouts of highly variable length outside the nest box (range 0.3–250 min, n  = 51), so prospecting small passerines would have difficulty predicting whether an aggressive tit owner was in the box or would soon return. We presented prospecting male pied flycatchers ( Ficedula hypoleuca ) with a dyad of boxes ( n  = 93), each containing a great tit nest but only one with visible eggs. Flycatchers hesitated more to enter a nest box with no visible tit eggs than a box with exposed eggs. This was most evident for nest boxes with dark versus light interior paint, supporting the idea that better interior illumination makes prospecting birds more confident about entering an unfamiliar cavity. The usurpation and predation hypotheses are not mutually exclusive because both competitors and small predators may hesitate to enter dark, enclosed spaces if visibility is low. Significance statement Some birds deposit a layer of material on top of the eggs when they leave the nest. Several hypotheses have been proposed for such egg covering, for example that it may insulate the eggs and reduce the risk of nest predation. We propose a new hypothesis, namely that secondary hole-nesting birds cover their eggs when they leave the nest to prevent usurpation of the cavity by other birds. Great tits that we filmed at the nest during the egg-laying period could be absent for long periods. To test the hypothesis, we presented male pied flycatchers, potential nest competitors, with a dyad of nest boxes, each containing a great tit nest but only one with visible tit eggs. In support of the prediction, prospecting flycatchers hesitated to enter dark cavities with dark floors relative to boxes with exposed, reflective eggs.

• Slagsvold, Tore & Wiebe, Karen (2021). Nest decoration: birds exploit a fear of feathers to guard their nest from usurpation. Royal Society Open Science. 8(11), p. 1–11. doi: 10.1098/rsos.211579. Full text in Research Archive Show summary

  Many species of birds incorporate feathers into their nest as structural support and to insulate the eggs or offspring. Here, we investigated the novel idea that birds reduce the risk of nest usurpation by decorating it with feathers to trigger a fear response in their rivals. We let prospecting birds choose between a dyad of nest-boxes in the wild, both containing some nest materials, but where one had a few white feathers and the other had none. All three species of cavity-nesting birds studied, the pied flycatcher Ficedula hypoleuca, the blue tit Cyanistes caeruleus, and the tree swallow Tachycineta bicolor, hesitated to enter boxes with white feathers. A similar avoidance of white feathers was found when the alternative nest-box of a dyad held black feathers. However, the birds readily collected white feathers that we placed in front of their nest-box, showing the fear of such feathers was context-dependent. We suggest that naive prospecting birds may perceive feathers in nests as the result of a predation event, and that owners decorate nests with bright feathers that can be seen from the opening to deter others from entering.

• Slagsvold, Tore & Wiebe, Karen L. (2021). Use of landmarks for nest site choice and small-scale navigation to the nest in birds. Behaviour. ISSN 0005-7959. 158(8-9), p. 705–726. doi: 10.1163/1568539X-bja10091. Full text in Research Archive Show summary

  Abstract Nest sites of animals are often concealed to keep vulnerable offspring from being detected by predators. Parents may use landmarks near the nest to relocate it quickly. We allowed blue tits ( Cyanistes caeruleus ) to choose between two nest boxes fixed on the same tree with either none, the same, or different white painted markings. Surprisingly, the female brought material to both boxes and sometimes laid eggs in both. In a second experiment, we let pied flycatchers ( Ficedula hypoleuca ) and great tits ( Parus major ) become familiar with a marking on the initial nest box and then let them choose between two new nest boxes erected on different, nearby trees. Neither species preferred the box with the matching mark. In nature, the birds may locate the correct entrance of a cavity using other landmarks near the nest opening, like branches and the height of the cavity opening above the ground.

• Slagsvold, Tore & Wiebe, Karen (2020). Interspecific aggression and defence of extra nest sites in two species of songbirds. Ethology. ISSN 0179-1613. 127, p. 294–301. doi: 10.1111/eth.13127. Full text in Research Archive Show summary

  Interspecific competition is expected when two species share resource needs. For example, secondary cavity‐nesting birds may compete vigorously for suitable nest sites both within and among species. However, little is known of whether monogamous species defend more than one nest site on their territory after breeding has begun, and in particular whether they are aggressive against other species. Defending extra nest sites may be adaptive because they may be used for renesting after a failure or to produce a second brood. We studied interactions between a monogamous, resident bird, the great tit Parus major and a migrant, the pied flycatcher Ficedula hypoleuca, providing nest boxes in woodlands in Norway. Agonistic behaviours may be subtle and easily overlooked so we experimentally placed caged male pied flycatchers near nests of great tits and at a dyad of empty nest boxes erected in the territory 25 m away. We filmed interactions between species at 21 sites in the egg‐laying period of the tit, and at 26 nest sites during incubation. Male great tits showed aggression towards the caged flycatcher both at their own nest box and at the nest boxes erected at a distance. We manipulated the external appearance of the nest boxes with painted designs around the entrances but the intensity of aggression at the empty nest boxes did not depend on whether those boxes matched the nest box with the tit nest and was not correlated with tit clutch size. Neither was the intensity of display activity at each nest box by the flycatchers that settled associated with these variables. The results are discussed in relation to hypotheses for nest site choice involving interspecific social learning and aggression.

• Møller, Anders Pape; Balbontin, Javier; Dhondt, André A.; Adriaensen, Frank; Artemyev, Alexandr & Banbura, Jerzy [Show all 71 contributors for this article] (2020). Interaction of climate change with effects of conspecific and heterospecific density on reproduction. Oikos. ISSN 0030-1299. 129, p. 1807–1819. doi: 10.1111/oik.07305. Full text in Research Archive
• Aasen, Maria & Slagsvold, Tore (2020). No cultural transmission of use of nest materials in titmice Paridae. Animal Behaviour. ISSN 0003-3472. 170, p. 27–32. doi: 10.1016/j.anbehav.2020.10.005. Full text in Research Archive Show summary

  Bird nests often consist of stronger materials to maintain nest structure and a softer layer of lining materials to reduce heat loss. We studied whether early learning affected the use of nest materials by cross-fostering between two tit species with similar breeding ecology, the blue tit, Cyanistes caeruleus, and the great tit, Parus major, in a woodland area provided with nestboxes in Norway. In both species, only the female builds the nest. From previous studies, we know that early social learning affects a number of traits in these birds, including song repertoire, mate choice, foraging behaviour and nest site choice. Nests of the two species are similar but blue tits use more feathers than great tits. This was confirmed in the present study; however, female blue tits raised by great tit foster parents also supplied their nest with feathers, and female great tits raised by blue tits used few feathers. The only treatment effect was that fewer feathers were found in nests of cross-fostered females in both species than in nests of controls. This may have been caused by time and energetic constraints during nest building because cross-fostered birds seemed to forage less efficiently than controls. The amount of hair was slightly greater in blue tits than in great tits, but it was not affected by the cross-fostering either. We conclude that no cultural transmission was found in the use of lining materials in the nest of the two species, perhaps because young birds hatch after their parents have stopped constructing the nest.

• Loukola, Olli J.; Adamik, Peter; Adriaensen, Frank; Barba, Emilio; Doligez, Blandine & Flensted‐Jensen, Einar [Show all 21 contributors for this article] (2020). The roles of temperature, nest predators and information parasites for geographical variation in egg covering behaviour of tits (Paridae). Journal of Biogeography. ISSN 0305-0270. 47, p. 1482–1493. doi: 10.1111/jbi.13830. Full text in Research Archive Show summary

  Aim Nest building is widespread among animals. Nests may provide receptacles for eggs, developing offspring and the parents, and protect them from adverse environmental conditions. Nests may also indicate the quality of the territory and its owner and can be considered as an extended phenotype of its builder(s). Nests may, thus, function as a sexual and social signal. Here, we examined ecological and abiotic factors—temperature, nest predation and interspecific information utilization—shaping geographical variation in a specific nest structure—hair and feather cover of eggs—and its function as an extended phenotype before incubation in great (Parus major) and blue tits (Cyanistes caeruleus) across Europe. We also tested whether egg covering is associated with reproductive success of great tits. Location Fourteen different study sites and 28 populations across Europe. Taxon Parus major, Cyanistes caeruleus. Methods We recorded clutch coverage estimates and collected egg covering nest material from the tit nests. We also measured nest specific breeding parameters and phenotypic measurements on adults. We tested whether mean spring temperatures, nest predation rates and flycatcher (Ficedula spp) densities in the study areas explain the large‐scale geographical variation of clutch coverage and reproductive success of tits. Results The degree of egg coverage of great tits increased with lower mean spring temperature, higher nest predation rate and higher flycatcher density. We did not find egg covering of blue tits to be associated with any of the ecological or abiotic factors. Moreover, egg covering of great tits was not associated with reproductive success in our cross‐sectional data, yet a rigorous assessment of fitness effects would require long‐term data. Main conclusions Our findings suggest that, in great tits, egg covering may simultaneously provide thermal insulation against cold temperatures during egg‐laying in spring and also represent a counter‐adaptation to reduce information parasitism by flycatchers and nest predation. Hence, geographical variation in interspecific interactions, and consequently in co‐evolutionary processes, may affect the evolution of nest characteristics besides environmental conditions.

• Culina, Antica; Adriaensen, Frank; Bailey, Liam D.; Burgess, Malcolm; Charmantier, Anne & Cole, Ella F [Show all 54 contributors for this article] (2020). Connecting the data landscape of long‐term ecological studies: The SPI‐Birds data hub. Journal of Animal Ecology. ISSN 0021-8790. p. 1–14. doi: 10.1111/1365-2656.13388. Full text in Research Archive Show summary

  The integration and synthesis of the data in different areas of science is drastically slowed and hindered by a lack of standards and networking programmes. Long‐term studies of individually marked animals are not an exception. These studies are especially important as instrumental for understanding evolutionary and ecological processes in the wild. Furthermore, their number and global distribution provides a unique opportunity to assess the generality of patterns and to address broad‐scale global issues (e.g. climate change). To solve data integration issues and enable a new scale of ecological and evolutionary research based on long‐term studies of birds, we have created the SPI‐Birds Network and Database (www.spibirds.org)—a large‐scale initiative that connects data from, and researchers working on, studies of wild populations of individually recognizable (usually ringed) birds. Within year and a half since the establishment, SPI‐Birds has recruited over 120 members, and currently hosts data on almost 1.5 million individual birds collected in 80 populations over 2,000 cumulative years, and counting. SPI‐Birds acts as a data hub and a catalogue of studied populations. It prevents data loss, secures easy data finding, use and integration and thus facilitates collaboration and synthesis. We provide community‐derived data and meta‐data standards and improve data integrity guided by the principles of Findable, Accessible, Interoperable and Reusable (FAIR), and aligned with the existing met

• Nilsson, Anna L. K.; Skaugen, Thomas; Reitan, Trond; L'Abee-Lund, Jan Henning; Gamelon, Marlène & Jerstad, Kurt [Show all 11 contributors for this article] (2020). Hydrology influences breeding time in the white-throated dipper. BMC Ecology. 20(1). doi: 10.1186/s12898-020-00338-y. Full text in Research Archive Show summary

  Abstract Background Earlier breeding is one of the strongest responses to global change in birds and is a key factor determining reproductive success. In most studies of climate effects, the focus has been on large-scale environmental indices or temperature averaged over large geographical areas, neglecting that animals are affected by the local conditions in their home ranges. In riverine ecosystems, climate change is altering the flow regime, in addition to changes resulting from the increasing demand for renewable and clean hydropower. Together with increasing temperatures, this can lead to shifts in the time window available for successful breeding of birds associated with the riverine habitat. Here, we investigated specifically how the environmental conditions at the territory level influence timing of breeding in a passerine bird with an aquatic lifestyle, the white-throated dipper Cinclus cinclus . We relate daily river discharge and other important hydrological parameters, to a long-term dataset of breeding phenology (1978–2015) in a natural river system. Results Dippers bred earlier when winter river discharge and groundwater levels in the weeks prior to breeding were high, and when there was little snow in the catchment area. Breeding was also earlier at lower altitudes, although the effect dramatically declined over the period. This suggests that territories at higher altitudes had more open water in winter later in the study period, which permitted early breeding also here. Unexpectedly, the largest effect inducing earlier breeding time was territory river discharge during the winter months and not immediately prior to breeding. The territory river discharge also increased during the study period. Conclusions The observed earlier breeding can thus be interpreted as a response to climate change. Measuring environmental variation at the scale of the territory thus provides detailed information about the interactions between organisms and the abiotic environment.

• Slagsvold, Tore (2019). Social Learning in Birds Studied by Cross-Fostering in the Wild. Integrative Journal of Veterinary Biosciences (IJVB). 3(2), p. 1–4. doi: 10.1098/rspb.2002.2045. Full text in Research Archive Show summary

  Social learning is widespread in nature and important for the behaviour, ecology and conservation of animals. We applied a method of cross-fostering between passerine birds in woodland areas in Norway, including great tits Parus major, blue tits Cyanistes caeruleus, and pied flycatchers Ficedula hypoleuca. When cross-fostering between tit species, the offspring sexually mis-imprinted on the foster species. Survival rate was similar to the controls but mating success was strongly reduced and mixed pairings occurred between cross-fostered birds of the two species, resulting in one case of hybridization. Song was affected, but the extent showed great variation among males. Male pied flycatchers raised by tits also included strophes from the foster species in their song, in particular if they had been raised together with siblings of the foster species. However, surprisingly, in flycatchers, mate choice was not affected by cross-fostering, indicating a different mechanism between song learning and mate choice. In tits, choice of nest site was affected by the cross-fostering, the birds taking characteristics of the natal nest site into account, and the behaviour of members of their foster parent species. Cross-fostering had a strong effect on foraging behaviour in terms of spatial location of foraging sites and the type of prey items provided to the young. Learning foraging techniques within the natal habitat may explain why prey provided by immigrant tits new to the study area differed from those provided by local recruits. We discuss how social learning may affect the evolution, behaviour and ecology of these birds, including the evolution of nest parasitism, and how cross-fostering among species may be used in management programs of endangered species.

• Nilsson, Anna; Slagsvold, Tore; Røstad, Ole Wiggo; Knudsen, Endre; Jerstad, Kurt & Cadahia, Luis [Show all 10 contributors for this article] (2019). Territory location and quality, together with climate, affect the timing of breeding in the white-throated dipper. Scientific Reports. 9, p. 1–11. doi: 10.1038/s41598-019-43792-5. Full text in Research Archive Show summary

  Recent climate change has led to advanced spring phenology in many temperate regions. The phenological response to variation in the local environment, such as the habitat characteristics of the territories birds occupy, is less clear. The aim of this study is to understand how ecological conditions affect breeding time, and its consequences for reproduction, in a white-throated dipper Cinclus cinclus population in a river system in Norway during 34 years (1978–2011). Hatching date advanced almost nine days, indicating a response to higher temperatures and the advanced phenology in the area. Earlier breeding was found in warm springs and at lower altitudes. High population density facilitated earlier breeding close to the coast. Furthermore, when population density was low, breeding was early at territories that were rarely occupied, while in years with high density, breeding was early at territories that were frequently occupied. Also, when population density was low, earlier breeding occurred at territories that on average produced more offspring than other territories, while there was no difference in breeding time in high population years. Selection for early breeding was dependent on spring temperatures and high spring temperatures contributed to higher breeding success during the study period. We found that breeding phenology may have strong effects on fitness in the white-throated dipper, and thus that breeding time is an important ecological factor in a species that feeds mainly on aquatic rather than terrestrial prey.

• Hogstad, Olav & Slagsvold, Tore (2018). Survival of Willow tits Poecile montanus: the significance of flock membership, social rank and body size. Ornis Norvegica. 41, p. 13–18. doi: 10.15845/on.v41i0.1540. Full text in Research Archive Show summary

  The Willow Tit Poecile montanus is highly sedentary and breeding pairs remain in their exclusive areas throughout the year. During the winter, these areas are defended by small, non-kin flocks, formed as the roaming yearlings become sedentary and join adults during late summer and autumn. Once established, stable social hierarchies are maintained in these flocks during the winter. The winter flocks consist normally of the socially dominant adult mated pair and two mated juvenile pairs, one higher-ranked and one lower-ranked. Individually colour-ringed juvenile Willow Tits were followed over years in subalpine forest in Norway from ringing in the autumn till they disappeared. None of the lower ranked birds survived their first winter, whereas only 4 of 71 higher-ranked juvenile pairs disappeared during this time. Half of the 71 pairs survived their first winter, about 25 % survived two winters, 8.5% survived three winters, and 5.6% survived four winters. Survival was similar for males and females. Alpha pairs remained mated and defended their common territory across years. Maximum age as revealed by ringing showed one female became six years old and two males ringed as adults were at least nine years old when last observed. The main factor associated with survival was early flock establishment that led to a high rank position among the juvenile flock members. Body size seemed insignificant. Birds that survived their first winter either succeeded to establish as territory owners or they were forced into the role as floaters and probably perished.

• Slagsvold, Tore & Wiebe, Karen (2018). Immigrants and locally recruited birds differ in prey delivered to their offspring in blue tits and great tits. Animal Behaviour. ISSN 0003-3472. 139, p. 127–135. doi: 10.1016/j.anbehav.2018.01.007. Full text in Research Archive Show summary

  Natal dispersal is common in animals but the fitness cost of moving from the natal area is not well understood. One reason for a fitness cost is that foraging skills and prey preference learned early in life may be less efficient if the individual settles in a new, unfamiliar habitat. In a 4-year study, we found that immigrant parent blue tits, Cyanistes caeruleus, and great tits, Parus major, were inferior food providers compared to local recruits. In blue tits, immigrants provided smaller prey items than local recruits, whereas in great tits, the immigrants provided fewer green larvae, but relatively more brown larvae, to the offspring than local recruits. We also found that immigrant females laid later or smaller clutches than females locally recruited. The results are consistent with the hypothesis that natal dispersal carries costs related to the learning of foraging skills. However, alternative explanations are that the differences were caused by genetic and/or quality differences between the two groups of birds. We discuss various ecological and behavioural traits that may influence, and be influenced by, the mismatch of foraging between natal and breeding habitats. In altricial birds, yearlings will not have previous foraging experience during breeding and, in addition, immigrants will not have spent a long postfledging period in the new local habitat with their parents. If there are foraging-habitat mismatches as a result of dispersal, researchers should include natal origin in models of optimal foraging, time budgets, reproductive success and survival because performance may be directly related to the early learning environment rather than genetic differences.

• Slagsvold, Tore & Wiebe, Karen (2018). On heterospecifc learning in birds – comments on Samplonius and Forsman et al. Journal of Avian Biology. ISSN 0908-8857. 49(3). doi: 10.1111/jav.01706. Full text in Research Archive Show summary

  In several papers, it has been argued that two migratory species, the pied and the collared flycatcher, Ficedula hypoleuca and F. albicollis, take clutch size of a resident bird, the great tit Parus major, into account when deciding which of two adjacent nest boxes to use for their own breeding attempt (Forsman et al. 2018). We termed this the interspecific cue hypothesis (the ICH; Slagsvold and Wiebe 2017) and argued that some of its basic assumptions have never been critically examined. Forsman et al. (2018) and Samplonius (2018) criticize our paper and argue that the ICH is the most plausible explanation but here we conclude that the new data they provide on the behaviour of the pied flycatcher (Forsman et al. 2018) are, in fact, inconsistent with the most critical assumption of the ICH. Therefore, the hypothesis should be rejected.

• Cadahia, Luis; Labra, Antonieta; Knudsen, Endre; Nilsson, Anna; Lampe, Helene M. & Slagsvold, Tore [Show all 7 contributors for this article] (2017). Advancement of spring arrival in a long‑term study of a passerine bird: sex, age and environmental effects. Oecologia. ISSN 0029-8549. 184(4), p. 917–929. doi: 10.1007/s00442-017-3922-4. Full text in Research Archive Show summary

  In migratory birds, mistimed arrival might have negative consequences for individual fitness, causing population declines. This may happen if arrival time is not synchronized with breeding time, especially when earlier springs favour earlier reproduction. We studied spring arrival time to the breeding areas in a pied flycatcher Ficedula hypoleuca population in southern Norway during a 30-year period (1985–2014). We investigated trends in arrival both for the entire population and for different population fractions (e.g. early vs. late arrivals). We also studied sex and age class differences, along with repeatability of arrival. Finally, we explored how arrival is influenced by environmental conditions at the areas birds use throughout the year, using mixed-effects models and quantile regressions with individual-based data. Spring arrival advanced over five days, at a similar rate through the entire population. Males and adult birds arrived earlier than females and yearlings. Arrival was significantly repeatable for males and females. Birds arrived earlier in years with high temperature and rainfall at the breeding grounds, and low NDVI both on the Iberian Peninsula and in central Europe. Later fractions of the population showed a steeper response to these environmental variables. This intra-population heterogeneity in the responses to the environment probably stems from a combination between the different selection pressures individuals are subject to and their age-related experience. Our results highlight the importance of studying how migration phenology is affected by the environment not only on the breeding grounds but also on the other areas birds use throughout the year.

• Gamelon, Marlène; Grøtan, Vidar; Nilsson, Anna L. K.; Engen, Steinar; Hurrell, James W. & Jerstad, Kurt [Show all 12 contributors for this article] (2017). Interactions between demography and environmental effects are important determinants of population dynamics. Science Advances. 3(2). doi: 10.1126/sciadv.1602298. Full text in Research Archive Show summary

  Climate change will affect the population dynamics of many species, yet the consequences for the long-term persistence of populations are poorly understood. A major reason for this is that density-dependent feedback effects caused by fluctuations in population size are considered independent of stochastic variation in the environment. We show that an interplay between winter temperature and population density can influence the persistence of a small passerine population under global warming. Although warmer winters favor an increased mean population size, density-dependent feedback can cause the local population to be less buffered against occasional poor environmental conditions (cold winters). This shows that it is essential to go beyond the population size and explore climate effects on the full dynamics to elaborate targeted management actions.

• Slagsvold, Tore & Wiebe, Karen (2017). On the use of heterospecific information for nest site selection in birds. Journal of Avian Biology. ISSN 0908-8857. 48(7), p. 1035–1040. doi: 10.1111/jav.01398. Full text in Research Archive Show summary

  This research has been accepted and published in the Journal of Avian Biology. © 2017 Wiley

• Cramer, Emily Rebecca A; Stensrud, Even; Marthinsen, Gunnhild; Hogner, Silje; Johannessen, Lars Erik & Laskemoen, Terje [Show all 10 contributors for this article] (2016). Sperm performance in conspecific and heterospecific female fluid. Ecology and Evolution. doi: 10.1002/ece3.1977. Full text in Research Archive Show summary

  Divergent sexual selection within allopatric populations may result in divergent sexual phenotypes, which can act as reproductive barriers between populations upon secondary contact. This hypothesis has been most tested on traits involved in precopulatory sexual selection, with less work focusing on traits that act after copulation and before fertilization (i.e., postcopulatory prezygotic traits), particularly in internally fertilizing vertebrates. However, postcopulatory sexual selection within species can also drive trait divergence, resulting in reduced performance of heterospecific sperm within the female reproductive tract. Such incompatibilities, arising as a by-product of divergent postcopulatory sexual selection in allopatry, can represent reproductive barriers, analogous to species-assortative mating preferences. Here, we tested for postcopulatory prezygotic reproductive barriers between three pairs of taxa with diverged sperm phenotypes and moderate-to-high opportunity for postcopulatory sexual selection (barn swallows Hirundo rustica versus sand martins Riparia riparia, two subspecies of bluethroats, Luscinia svecica svecica versus L. s. namnetum, and great tits Parus major versus blue tits Cyanistes caeruleus). We tested sperm swimming performance in fluid from the outer reproductive tract of females, because the greatest reduction in sperm number in birds occurs as sperm swim across the vagina. Contrary to our expectations, sperm swam equally well in fluid from conspecific and heterospecific females, suggesting that postcopulatory prezygotic barriers do not act between these taxon pairs, at this stage between copulation and fertilization. We therefore suggest that divergence in sperm phenotypes in allopatry is insufficient to cause widespread postcopulatory prezygotic barriers in the form of impaired sperm swimming performance in passerine birds.

• Vaugoyeau, Marie; Adriaensen, Frank; Artemyev, Alexandr; Banbura, Jerzy; Barba, Emilio & Biard, Clotilde [Show all 77 contributors for this article] (2016). Interspecific variation in the relationship between clutch size, laying date and intensity of urbanization in four species of hole-nesting birds. Ecology and Evolution. 6(16), p. 5907–5920. doi: 10.1002/ece3.2335. Full text in Research Archive
• Laine, Veronika N.; Gossmann, Toni; Schachtschneider, Kyle M.; Garroway, Colin J.; Madsen, Ole & Verhoeven, Koen J.F. [Show all 46 contributors for this article] (2016). Evolutionary signals of selection on cognition from the great tit genome and methylome. Nature Communications. 7. doi: 10.1038/ncomms10474. Full text in Research Archive Show summary

  For over 50 years, the great tit (Parus major) has been a model species for research in evolutionary, ecological and behavioural research; in particular, learning and cognition have been intensively studied. Here, to provide further insight into the molecular mechanisms behind these important traits, we de novo assemble a great tit reference genome and whole-genome re-sequence another 29 individuals from across Europe. We show an overrepresentation of genes related to neuronal functions, learning and cognition in regions under positive selection, as well as increased CpG methylation in these regions. In addition, great tit neuronal non-CpG methylation patterns are very similar to those observed in mammals, suggesting a universal role in neuronal epigenetic regulation which can affect learning-, memory- and experience-induced plasticity. The high-quality great tit genome assembly will play an instrumental role in furthering the integration of ecological, evolutionary, behavioural and genomic approaches in this model species.

• Lemoine, Melissa; Lucek, Kay; Perrier, Charles; Saladin, Verena; Adriaensen, Frank & Barba, Emilio [Show all 27 contributors for this article] (2016). Low but contrasting neutral genetic differentiation shaped by winter temperature in European great tits. Biological Journal of the Linnean Society. ISSN 0024-4066. 118(3), p. 668–685. doi: 10.1111/bij.12745. Full text in Research Archive
• Wiebe, Karen & Slagsvold, Tore (2015). Foraging trade-offs between prey size, delivery rate and prey type: How does niche breadth and early learning of the foraging niche affect food delivery? Ethology. ISSN 0179-1613. 121(10), p. 1010–1017. doi: 10.1111/eth.12411. Full text in Research Archive
• Husek, Jan; Lampe, Helene M. & Slagsvold, Tore (2014). Natal dispersal based on past and present environmental phenology in the pied flycatcher (Ficedula hypoleuca). Oecologia. ISSN 0029-8549. 174(4), p. 1139–1149. doi: 10.1007/s00442-013-2842-1. Full text in Research Archive Show summary

  Natal dispersal allows individuals to reach suitable breeding sites. The effect of present plant phenology as a cue for dispersal into areas with favourable stages of development has been well established across avian and mammalian taxa. However, the effect of past experience is less understood. We studied the effect of past and present phenology of the environment on the direction and distance of natal dispersal in a passerine bird, the pied flycatcher (Ficedula hypoleuca). We monitored spring settlement of local recruits in six nest box plots along a 10-km stretch of a south-north gradient of plant and caterpillar food development. We found that males used both past experience of caterpillar phenology from early life and actual plant phenology during the recruitment season as independent cues for breeding settlement. Males that had experienced a mismatch with the caterpillar food peak as a nestling, and/or those that arrived late in the spring in the recruitment year, moved north of their natal site, whereas males that had experienced a better match with the caterpillars as a nestling, and/or those that migrated earlier in the spring, settled at a similar site or more to the south. In females, no such effects were found, suggesting that the usage of phenological cues is sex specific. In summary, tracking environmental phenology by natal dispersal may represent an effective mechanism for settling in new favourable areas, and may thus potentially cause rapid change of a species’ geographical breeding range in response to climate change.

• Sonerud, Geir Andreas; Steen, Ronny; Løw, Line Marie; Røed, Line T.; Skar, Kristin & Selås, Vidar [Show all 7 contributors for this article] (2014). Evolution of parental roles in raptors: Prey type determines role asymmetry in the Eurasian kestrel. Animal Behaviour. ISSN 0003-3472. 96, p. 31–38. doi: 10.1016/j.anbehav.2014.07.011. Full text in Research Archive
• Møller, Anders P.; Adriaensen, Frank; Artemyev, Alexandr; Banbura, Jerzy; Barba, Emilio & Biard, Clotilde [Show all 82 contributors for this article] (2014). Variation in clutch size in relation to nest size in birds. Ecology and Evolution. 4(18), p. 3583–3595. doi: 10.1002/ece3.1189. Full text in Research Archive
• Slagsvold, Tore; Husek, Jan; Whittington, Jason & Wiebe, Karen (2014). Antipredator behavior: Escape flights on a landscape slope. Behavioral Ecology. ISSN 1045-2249. 25(2), p. 378–385. doi: 10.1093/beheco/aru003. Full text in Research Archive
• Sonerud, Geir Andreas; Steen, Ronny; Selås, Vidar; Aanonsen, Ole Martin; Aasen, Gunn-Henny & Fagerland, Kristine L. [Show all 21 contributors for this article] (2014). Evolution of parental roles in provisioning birds: Diet determines role asymmetry in raptors. Behavioral Ecology. ISSN 1045-2249. 25(4), p. 762–772. doi: 10.1093/beheco/aru053. Full text in Research Archive
• Wiebe, Karen L. & Slagsvold, Tore (2014). Prey size increases with nestling age: Are provisioning parents programmed or responding to cues from offspring? Behavioral Ecology and Sociobiology. ISSN 0340-5443. 68(5), p. 711–719. doi: 10.1007/s00265-014-1684-0. Full text in Research Archive
• Møller, Anders Pape; Adriaensen, Frank; Artemyev, Alexandr; Banbura, Jerzy; Barba, Emilio & Biard, Clotilde [Show all 86 contributors for this article] (2014). Clutch-size variation in Western Palaearctic secondary hole-nesting passerine birds in relation to nest box design. Methods in Ecology and Evolution. 5(4), p. 353–362. doi: 10.1111/2041-210X.12160. Full text in Research Archive
• Slagsvold, Tore; Kleiven, Kari; Eriksen, Ane & Johannessen, Lars Erik (2013). Vertical and horizontal transmission of nest site preferences in titmice. Animal Behaviour. ISSN 0003-3472. 85(2), p. 323–328. doi: 10.1016/j.anbehav.2012.10.024. Full text in Research Archive
• Slagsvold, Tore; Eriksen, Ane; Ayala, Rosa Mary de; Husek, Jan & Wiebe, Karen (2013). POSTFLEDGING MOVEMENTS IN BIRDS: DO TIT FAMILIES TRACK ENVIRONMENTAL PHENOLOGY? The AUK: A Quarterly Journal of Ornithology. ISSN 0004-8038. 130(1), p. 36–45. doi: 10.1525/auk.2012.12122. Full text in Research Archive
• Møller, Anders Pape; Merino, Santiago; Soler, Juan José; Antonov, Anton; Badás, Elisa P. & Calero-Torralbo, Miguel A. [Show all 37 contributors for this article] (2013). Assessing the effects of climate on host-parasite interactions: a comparative study of European birds and their parasites. PLOS ONE. 8(12). doi: 10.1371/journal.pone.0082886. Full text in Research Archive Show summary

  Background: Climate change potentially has important effects on distribution, abundance, transmission and virulence of parasites in wild populations of animals. Methodology/Principal Finding: Here we analyzed paired information on 89 parasite populations for 24 species of bird hosts some years ago and again in 2010 with an average interval of 10 years. The parasite taxa included protozoa, feather parasites, diptera, ticks, mites and fleas. We investigated whether change in abundance and prevalence of parasites was related to change in body condition, reproduction and population size of hosts. We conducted analyses based on the entire dataset, but also on a restricted dataset with intervals between study years being 5–15 years. Parasite abundance increased over time when restricting the analyses to datasets with an interval of 5–15 years, with no significant effect of changes in temperature at the time of breeding among study sites. Changes in host body condition and clutch size were related to change in temperature between first and second study year. In addition, changes in clutch size, brood size and body condition of hosts were correlated with change in abundance of parasites. Finally, changes in population size of hosts were not significantly related to changes in abundance of parasites or their prevalence. Conclusions/Significance: Climate change is associated with a general increase in parasite abundance. Variation in laying date depended on locality and was associated with latitude while body condition of hosts was associated with a change in temperature. Because clutch size, brood size and body condition were associated with change in parasitism, these results suggest that parasites, perhaps mediated through the indirect effects of temperature, may affect fecundity and condition of their hosts. The conclusions were particularly in accordance with predictions when the restricted dataset with intervals of 5– 15 years was used, suggesting that short intervals may bias findings.

• Landys, Meta M.; Goymann, Wolfgang; Soma, Kiran K. & Slagsvold, Tore (2013). Year-round territorial aggression is independent of plasma DHEA in the European nuthatch Sitta europaea. Hormones and Behavior. ISSN 0018-506X. 63(1), p. 166–172. doi: 10.1016/j.yhbeh.2012.10.002. Full text in Research Archive
• Sonerud, Geir Andreas; Steen, Ronny; Løw, Line Marie; Røed, Line T.; Skar, Kristin & Selås, Vidar [Show all 7 contributors for this article] (2013). Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors. Oecologia. ISSN 0029-8549. 172(1), p. 93–107. doi: 10.1007/s00442-012-2491-9. Full text in Research Archive
• Wiebe, Karen & Slagsvold, Tore (2012). Parents take both size and conspicuousness into account when feeding nestlings in dark cavity nests. Animal Behaviour. ISSN 0003-3472. 84(6), p. 1307–1312. doi: 10.1016/j.anbehav.2012.09.014. Full text in Research Archive
• Steen, Ronny; Sonerud, Geir Andreas & Slagsvold, Tore (2012). Parents adjust feeding effort in relation to nestling age in the Eurasian Kestrel (Falco tinnunculus). Journal of Ornithology = Journal fur Ornithologie. ISSN 0021-8375. 153(4), p. 1087–1099. doi: 10.1007/s10336-012-0838-y. Full text in Research Archive
• Lifjeld, Jan Terje; Laskemoen, Terje; Kleven, Oddmund; Pedersen, Anne Tiril Myhre; Lampe, Helene M. & Rudolfsen, Geir [Show all 8 contributors for this article] (2012). No Evidence for Pre-Copulatory Sexual Selection on Sperm Length in a Passerine Bird. PLOS ONE. 7(2). doi: 10.1371/journal.pone.0032611. Full text in Research Archive Show summary

  There is growing evidence that post-copulatory sexual selection, mediated by sperm competition, influences the evolution of sperm phenotypes. Evidence for pre-copulatory sexual selection effects on sperm traits, on the other hand, is rather scarce. A recent paper on the pied flycatcher, Ficedula hypoleuca, reported phenotypic associations between sperm length and two sexually selected male traits, i.e. plumage colour and arrival date, thus invoking pre-copulatory sexual selection for longer sperm. We were unable to replicate these associations with a larger data set from the same and two additional study populations; sperm length was not significantly related to either male plumage colour or arrival date. Furthermore, there was no significant difference in sperm length between populations despite marked differences in male plumage colour. We also found some evidence against the previously held assumption of longer sperm being qualitatively superior; longer sperm swam at the same speed as shorter sperm, but were less able to maintain speed over time. We argue that both empirical evidence and theoretical considerations suggest that the evolution of sperm morphology is not primarily associated with pre-copulatory sexual selection on male secondary sexual traits in this or other passerine bird species. The relatively large between-male variation in sperm length in this species is probably due to relaxed post-copulatory sexual selection.

• Husek, Jan; Weidinger, Karel; Adamik, Peter & Slagsvold, Tore (2012). Impact of Nesting Mortality on Avian Breeding Phenology: A Case Study on the Red-Backed Shrike (Lanius collurio). PLOS ONE. 7(8:e43944). doi: 10.1371/journal.pone.0043944. Full text in Research Archive Show summary

  The seasonal timing of avian reproduction is supposed primarily to coincide with favourable feeding conditions. Long-term changes in avian breeding phenology are thus mostly scrutinized in relation to climatic factors and matching of the food supplies, while the role of nesting mortality is largely unexplored. Here we show that higher seasonal mean daily mortality rate leads to a shift in the distribution of breeding times of the successful nests to later dates in an an open-nesting passerine bird, the red-backed shrike Lanius collurio. The effect appeared to be strong enough to enhance or counteract the influence of climatic factors and breeding density on the inter-annual variation in mean hatching dates. Moreover, the seasonal distribution of reproductive output was shifted to larger, or smaller, broods early in the season when the nesting mortality increased, or decreased, respectively, during the season. We suggest that population level changes in timing of breeding caused by a general advancement of spring and of the food supplies might be altered by the seasonality in nesting mortality. Hence, we argue that consideration of nesting mortality is of major importance for understanding long-term trends in avian phenology, particularly in species capable of renesting.

• Burger, Claudia; Belskii, E; Eeva, T; Laaksonen, T; Mägi, M & Mand, Raivo [Show all 14 contributors for this article] (2012). Climate change, breeding date and nestling diet: how temperature differentially affects seasonal changes in pied flycatcher diet depending on habitat variation. Journal of Animal Ecology. ISSN 0021-8790. 81(4), p. 926–936. doi: 10.1111/j.1365-2656.2012.01968.x. Full text in Research Archive
• Wiebe, Karen & Slagsvold, Tore (2012). Brood parasites may use gape size constraints to exploit provisioning rules of smaller hosts: an experimental test of mechanisms of food allocation. Behavioral Ecology. ISSN 1045-2249. 23(2), p. 391–396. doi: 10.1093/beheco/arr202. Full text in Research Archive
• Johnsen, Arild; Vesterkjær, Kristin & Slagsvold, Tore (2011). Do Male Pied Flycatchers (Ficedula hypoleuca) Adjust Their Feeding Effort According to Egg Colour? Ethology. ISSN 0179-1613. 117(4), p. 309–317. doi: 10.1111/j.1439-0310.2011.01876.x. Full text in Research Archive
• Saggese, Katja; Korner-Nievergelt, Fränzi; Slagsvold, Tore & Amrhein, Valentin (2011). Wild bird feeding delays start of dawn singing in the great tit. Animal Behaviour. ISSN 0003-3472. 81(2), p. 361–365. doi: 10.1016/j.anbehav.2010.11.008. Full text in Research Archive
• Nilsson, Anna; Knudsen, Endre; Jerstad, Kurt; Røstad, Ole Wiggo; Walseng, Bjørn & Slagsvold, Tore [Show all 7 contributors for this article] (2011). Climate effects on population fluctuations of the white-throated dipper Cinclus cinclus. Journal of Animal Ecology. ISSN 0021-8790. 80(1), p. 235–243. doi: 10.1111/j.1365-2656.2010.01755.x. Full text in Research Archive
• Eriksen, Ane; Slagsvold, Tore & Lampe, Helene M. (2011). Vocal Plasticity - are Pied Flycatchers, Ficedula Hypoleuca, Open-Ended Learners? Ethology. ISSN 0179-1613. 117(3), p. 188–198. doi: 10.1111/j.1439-0310.2010.01864.x. Full text in Research Archive
• Landys, Meta M.; Goymann, Wolfgang & Slagsvold, Tore (2011). Rearing conditions have long-term consequences for stress responsiveness in free-living great tits. General and Comparative Endocrinology. ISSN 0016-6480. 174(2), p. 219–224. doi: 10.1016/j.ygcen.2011.08.022. Full text in Research Archive
• Steen, Ronny; Løw, Line Marie; Sonerud, Geir Andreas; Selås, Vidar & Slagsvold, Tore (2011). Prey delivery rates as estimates of prey consumption by Eurasian Kestrel Falco tinnunculus nestlings. Ardea. ISSN 0373-2266. 99(1), p. 1–8. doi: 10.5253/078.099.0101. Full text in Research Archive
• Slagsvold, Tore & Wiebe, Karen (2011). Social learning in birds and its role in shaping a foraging niche. Philosophical Transactions of the Royal Society of London. Biological Sciences. ISSN 0962-8436. 366(1567), p. 969–977. doi: 10.1098/rstb.2010.0343. Full text in Research Archive
• Størmer, Fredrik; Mysterud, Ivar & Slagsvold, Tore (2011). Evolution and possible storage of information in a magnetite system of significance for brain development. Medical Hypotheses. ISSN 0306-9877. 76(6), p. 901–904. doi: 10.1016/j.mehy.2011.03.004. Full text in Research Archive
• Nilsson, Anna; Stige, Leif Christian; Jerstad, Kurt; Røstad, Ole Wiggo; Slagsvold, Tore & Knudsen, Endre [Show all 8 contributors for this article] (2011). To make the most of what we have: extracting phenological data from nestling measurements. International journal of biometeorology. ISSN 0020-7128. 55, p. 797–804. doi: 10.1007/s00484-011-0461-2. Full text in Research Archive
• Turtumøygard, Tea & Slagsvold, Tore (2010). Evolution of brood parasitism in birds: constraints related to prey type. Behaviour. ISSN 0005-7959. 147(3), p. 299–317. doi: 10.1163/000579509X12529334990189. Full text in Research Archive
• Lambrechts, M.; Adriaensen, Frank; Ardia, Daniel R.; Artemyev, AV; Atienzar, Francisco & Banbura, Jerzy [Show all 56 contributors for this article] (2010). The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases. Acta Ornithologica. ISSN 0001-6454. 45(1), p. 1–26. doi: 10.3161/000164510X516047. Full text in Research Archive
• Landys, Meta M.; Goymann, W.; Schwabl, Ingrid; Trapschuh, Monika & Slagsvold, Tore (2010). Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird. Hormones and Behavior. ISSN 0018-506X. 58(2), p. 317–325. doi: 10.1016/j.yhbeh.2010.02.013. Full text in Research Archive
• Slagsvold, Tore; Sonerud, Geir Andreas; Grønlien, Helge E & Stige, Leif Christian (2010). Prey handling in raptors in relation to their morphology and feeding niches. Journal of Avian Biology. ISSN 0908-8857. 41(4), p. 488–497. doi: 10.1111/j.1600-048X.2010.05081.x. Full text in Research Archive
• Hansen, Bo Terning; Johannessen, Lars Erik & Slagsvold, Tore (2010). Interspecific cross-fostering of great tits (Parus major) by blue tits (Cyanistes caeruleus) affects inter- and intraspecific communication. Behaviour. ISSN 0005-7959. 147(3), p. 413–424. doi: 10.1163/000579509X12578482434926. Full text in Research Archive
• Steen, Ronny; Løw, Line Marie; Sonerud, Geir Andreas; Selås, Vidar & Slagsvold, Tore (2010). The feeding constraint hypothesis: prey preparation as a function of nestling age and prey mass in the Eurasian kestrel. Animal Behaviour. ISSN 0003-3472. 80(1), p. 147–153. doi: 10.1016/j.anbehav.2010.04.015. Full text in Research Archive Show summary

  The feeding constraint hypothesis states that an inability of young nestlings to ingest prey included in the diet of older nestlings and adult birds affects the evolution of parental behaviour, and predicts that the extent of prey preparation would increase with prey size and decrease with nestling age. In the Eurasian kestrel, Falco tinnunculus, a small raptor with a wide diet, parents often prepare prey prior to delivery at the nest, most notably by decapitation. We studied this behaviour by video monitoring prey deliveries at 29 nests for a total of about 200 days over 3 years. The probability of a prey item being decapitated prior to delivery differed between prey types and prey sizes; none of the insects or common lizards, Zootoca vivipara, and almost none of the shrews (Soricidae) were decapitated, whereas voles (Microtinae) and birds were commonly decapitated. For voles the probability of being decapitated decreased with nestling age, which supports the feeding constraint hypothesis because the nestlings' gape size limit and swallowing capacity would increase with age. For both voles and birds the probability of being decapitated increased with prey body mass, suggesting that kestrel nestlings may be unable to swallow, digest or egest skulls from larger prey. We suggest that the extent to which kestrel parents decapitate prey prior to delivery is an effect of their nestlings' age-dependent swallowing capacity and that the age of the nestlings therefore imposes constraints on the kestrel parents' foraging behaviour. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

View all works in NVA

• Slagsvold, Tore & Wiebe, Karen (2025). On heterospecific learning in birds – response to Hämäläinen et al. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 79(9). doi: 10.1007/s00265-025-03622-3. Full text in Research Archive
• Lifjeld, Jan Terje & Slagsvold, Tore (2022). Female extra-pair copulation for direct fitness benefits: A cautionary note. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 119(30). doi: 10.1073/pnas.2207108119. Full text in Research Archive
• Nilsson, Anna L.K.; Reitan, Trond; Skaugen, Thomas; L'Abee-Lund, Jan Henning; Gamelon, Marlène & Jerstad, Kurt [Show all 11 contributors for this article] (2020). Location Is Everything, but Climate Gets a Share: Analyzing Small-Scale Environmental Influences on Breeding Success in the White-Throated Dipper. Frontiers in Ecology and Evolution. doi: 10.3389/fevo.2020.542846. Full text in Research Archive Show summary

  Ecological and evolutionary effects of environmental variation on wild populations are of particular interest in a changing world. Large-scale environmental indices are classically used as environmental explanatory variables to study climate change effects on wild populations. However, these measures neglect the fact that individuals are affected by the local conditions in their home ranges. Freshwater ecosystems are particularly sensitive to climate change induced alterations in precipitation and discharge affecting lower trophic level prey and cascading up the food chain to impact keystone species. Here, we study how small-scale environmental variation at the level of each territory affects fitness-related traits and in particular reproductive success in the white-throated dipper Cinclus cinclus, a bird dependent on aquatic prey. We take advantage of a long-term study of breeding dippers in a natural river system in Norway (1978–2015). We relate daily river discharge and other important hydrological and territory location variables to clutch and brood size simultaneously, while accounting for male and female age, polygyny, and population density. We also address uncertainty in estimates of both clutch and brood size when modeling reproductive success. The most influential variable overall was the altitudinal gradient, where birds breeding at higher altitudes laid fewer eggs and fledged fewer young. Clutch size decreased with increased population density and high temperatures, while it was positively affected by female age. Brood size varied greatly among territories, where some territories had consistently larger broods than others. Increased minimum and maximum temperatures, river discharge above 10 l/s and increased variability in snow depth had a positive effect on brood size, whereas polygyny had a negative effect. In conclusion, territory-specific variables explained a large amount of the variation in the reproductive output in the dipper, emphasizing the importance of the local habitat, and not only climatic variation, in a keystone species in freshwater ecosystems.

• Aandstad, Kristian & Slagsvold, Tore (2017). Crystallization of song in the pied flycatcher (Ficedula hypoleuca). Full text in Research Archive
• Hol, Kristiane E. & Slagsvold, Tore (2017). On the “silver-spoon” effect: Telomere length in relation to rearing conditions in blue tits. Full text in Research Archive
• MacNeish, Avery & Slagsvold, Tore (2017). Variation in telomere length in blue tits in relation to age, sex, breeding success, and the telomere length of their parents. Full text in Research Archive
• Fong, Lydia & Slagsvold, Tore (2017). Comparing the song variation of early and late arrivals of pied flycatchers (Ficedula hypoleuca). Full text in Research Archive
• Slagsvold, Tore (2017). On the use of heterospecific information in birds. Full text in Research Archive
• Fong, Lydia & Slagsvold, Tore (2016). Comparing the song variation of early and late arrivals of juvenile pied flycatchers (Ficedula hypoleuca). Full text in Research Archive
• Aandstad, Kristian & Slagsvold, Tore (2016). Crystallization of song in the pied flycatcher (Ficedula hypoleuca). Full text in Research Archive
• Hol, Kristiane E. & Slagsvold, Tore (2016). On the “silver-spoon” effect: telomere length in relation to rearing conditions in blue tits. Full text in Research Archive
• MacNeish, Avery & Slagsvold, Tore (2016). Variation in telomere length in blue tits in relation to age, sex, breeding success, and the telomere length of their parents. Full text in Research Archive
• Slagsvold, Tore (2015). On the significance of early social learning in titmice. Full text in Research Archive
• Skaraas, Christine & Slagsvold, Tore (2015). Personality and natal dispersal in great tits Parus major. Full text in Research Archive
• Thorsteinsen, Camilla; Slagsvold, Tore & Lampe, Helene M. (2014). Reproductive success in great tits (Parus major) in relation to individual personality. Full text in Research Archive
• Skaraas, Christine; Slagsvold, Tore & Lampe, Helene M. (2014). Personality and natal dispersal in great tits Parus major. Full text in Research Archive
• Hagelin, Jon Valbjørn; Ergon, Torbjørn & Slagsvold, Tore (2014). Change in distribution of Norwegian alpine breeding birds in response to global warming. Full text in Research Archive
• Stræte, Tina Ø.; Slagsvold, Tore & Lampe, Helene M. (2014). Consistency between personality traits in wild great tits Parus major. Full text in Research Archive
• Thorsteinsen, Camilla & Slagsvold, Tore (2013). Winter survival and reproductive success in great tits in relation to individual personality. Full text in Research Archive
• Ivarsen, Anne E. & Slagsvold, Tore (2013). Does brain size affect survival and reproductive success in a wild population of great tits Parus major? Full text in Research Archive
• Stræte, Tina Ø. & Slagsvold, Tore (2013). Significance of rearing conditions for personality later in life: a field study of great tits Parus major. Full text in Research Archive
• Slagsvold, Tore (2012). Social learning in birds studied by cross-fostering in the wild. Full text in Research Archive
• Slagsvold, Tore (2012). Citizen science. Full text in Research Archive
• Slagsvold, Tore (2011). Feeding garden birds such as tits delays dawn chorus. [Journal]. BBC Earth News. Full text in Research Archive
• Slagsvold, Tore (2011). Bird feeders delay great tits' dawn chorus. [Internet]. NewScientist. Full text in Research Archive
• Slagsvold, Tore (2010). Det biologiske mennesket – individer og samfunn i lys av evolusjon. [Internet]. Nina.no. Full text in Research Archive
• Eriksen, Ane & Slagsvold, Tore (2010). Are mothers models for song learning - an experimental study on great tits. Full text in Research Archive Show summary

  In songbirds, it is assumed that males learn songs from their father and conspecific male neighbours. We studied if the mother may play a role as well. Great tits Parus major were experimentally cross-fostered to blue tits Cyanistes caeruleus in the wild, and vice versa. In both species, cross-fostered males sang a combination of conspecific and heterospecific song, demonstrating an effect of early learning. Due to sexual misimprinting, cross-fostering resulted in pairings between the two species, providing an opportunity to see how species and gender of parent influence song learning. Great tit males reared by a great tit male and a blue tit female included blue tit song in their repertoires. To control for the possibility that they had learned blue tit songs from their cross-fostered great tit fathers, males reared by great tit pairs in which the father had been cross-fostered served as controls. None of these sang blue tit song, revealing an effect of the social mother on song acquisition. In great tits, the female rarely sings, and so it is possible that the juveniles select the male neighbours to be used as song tutors based on the neighbours’ resemblance to their (foster) mother as well as father.

• Husek, Jan; Slagsvold, Tore; Weidinger, Karel & Adamik, Peter (2010). Impact of nest predation on breeding phenology in birds. Full text in Research Archive
• Slagsvold, Tore (2010). Feeding the birds can delay their dawn chorus. [Journal]. The Telegraph. Full text in Research Archive
• Slagsvold, Tore (2010). El dilema de alimentar a las aves silvestres. [Internet]. BBC Mundo. Full text in Research Archive
• Slagsvold, Tore (2010). Don't feed the birds. [Journal]. The Economist. Full text in Research Archive
• Eriksen, Ane; Slagsvold, Tore & Lampe, Helene M. (2011). Song learning in oscine songbirds : Tutor choice, timing, and the relationship with sexual imprinting. Universitetet i Oslo. Full text in Research Archive Show summary

  Birdsong is a species-specific signal that is used in mate attraction and intrasexual competition. This thesis concerns song acquisition in two oscine songbird species, the great tit Parus major and the pied flycatcher Ficedula hypoleuca. It addresses questions such as how free-living songbirds choose their song tutors and whether learning of foreign songs can take place in adult birds, and it looks into the relationship between song learning and sexual imprinting. This was done using a method of experimental, interspecific cross-fostering, in which eggs were swapped between nests of different species. Great tits were cross-fostered to blue tits Cyanistes caeruleus, and pied flycatchers were cross-fostered to great tits and blue tits. In oscine songbirds, young males have to learn their song from adult conspecifics, and a mechanism is needed to ensure that they choose song tutors of the right species. Early laboratory studies proposed the existence of an innate auditory template facilitating the recognition and memorization of conspecific song. However, in a number of species, males reared by heterospecifics have been found to copy songs of the foster species and thereby become mixed singers. To account for such heterospecific song learning, it has been suggested that familiarization with the social father before independence guides the choice of song tutors. However, under natural conditions, young males may grow up without a social father. A study on captive zebra finches Taeniopygia guttata has demonstrated effects of the social mother on song learning. Based on this finding, I propose a mechanism of mothermediated tutor choice, suggesting that experience with the social mother as well as the father guides the choice of song tutor in oscine songbirds. Great tits cross-fostered to blue tits include blue tit song in their repertoires. They also get sexually misimprinted, and occasional heterospecific pairs are formed between cross-fostered great tits and cross-fostered blue tits. I tested the hypothesis of mother-mediated tutor choice by letting great tits grow up with a blue tit foster mother and a great tit foster father. Males of this treatment included blue tit song in their repertoires. To control for the possibility that the males had learned the blue tit song directly from their cross-fostered fathers, we also recorded song from great tit males that were reared by conspecifics, but whose fathers had been cross-fostered to blue tits. None of these males sang blue tit song, suggesting an effect of the social mother. However, cross-fostered great tit males mated to blue tits may sing more blue tit song than those mated to great tits. Our samples were too small to separate between heterospecific song learning resulting from mother-mediated tutor choice and from exposure to blue tit song in the repertoire of the father. Song learning shares a number of characteristics with sexual imprinting. I investigated the relationship between these two processes in great tits. I exposed cross-fostered great tit males to playback of great tit song and blue tit song inside their territory. Males that showed sexual interest in blue tit females were more likely to respond with blue tit song to playback than males that showed interest only in conspecific females. This suggests that different degrees of misimprinting may have parallel effects on song learning and sexual preferences in cross-fostered great tit males. Furthermore, the response to territorial intruders seemed to be affected by social interactions: Cross-fostered males currently associated with blue tits increased their response to playback of blue tit song whereas males currently associated with great tits increased their response to playback of great tit song through the season. I also studied the effect of social rearing condition on song learning in pied flycatchers. Cross-fostered pied flycatchers do not seem to get sexually misimprinted. However, I found that pied flycatcher males reared by blue tits or great tits became mixed singers, uttering a combination of pied flycatcher song and the foster species’ song. Furthermore, males reared with nest mates of the foster species included tit song in a higher proportion of strophes than did cross-fostered males reared with conspecific nest mates. Hence, in pied flycatchers, growing up with heterospecific foster parents and nest mates caused a mixed choice of song tutors without affecting their mate preferences. Some songbird species are open-ended learners and retain the ability to learn new songs throughout their lives. However, to demonstrate that a species is a true open-ended learner, it is necessary to make sure that the elements learned as adults have not been heard and possibly memorized by the birds at an early age. I performed a playback experiment to test whether the pied flycatcher is a true open-ended learner. I exposed pied flycatcher males to playback of song containing syllables that were unknown to the study area. Among the 20 subjects, one yearling and two older males had learned a foreign syllable type, supporting the hypothesis that pied flycatchers are true open-ended learners. Pied flycatcher song is believed to serve mostly for mate attraction, and possible functions of adult song learning are to increase the repertoire size and to increase syllable sharing with neighbours. The subjects showed no increase in either of these parameters after the treatment. An alternative adaptive value of adult song learning in pied flycatchers may rather be to pick up particularly attractive song elements.

View all works in NVA