High colour variability in animals has been associated with adaptability to environmental change. Establishing how such variability is distributed in time and space, and identifying environmental correlates, can help understand the processes driving it. Common Buzzards Buteo buteo (herein buzzards) show extensive variation in plumage colour, ranging from dark brown to largely white, yet the factors maintaining or underlying this variation remain poorly understood. Previous studies showed that intermediate-coloured buzzards have higher fitness and have increased in abundance over time in a Dutch population. We compiled a large citizen science dataset to map buzzard colour variation across Europe, assess temporal changes and test for potential environmental drivers. Buzzard colour variation is geographically structured: light-coloured buzzards are mainly found in North-Central Europe, intermediates in South-East Europe and the British Isles, and darker buzzards in Iberia and Brittany. Lighter coloured buzzards are found in areas with lower temperatures and UVB radiation, and few volcanic soils (andosols), while darker birds are more common in forested areas. Between 2000 and 2022, the proportion of dark- and light-coloured buzzards declined, while intermediates increased, reducing colour variation over time. The temporal decrease in light-coloured buzzards is stronger in regions with a strong increase in temperature. Conversely, the decrease of dark morphs and the increase in intermediates are steeper where forest cover declined. Our results suggest that photoprotection and camouflage can partly explain spatial variation in buzzard colour, and that sulphur-rich soils may foster increased pigmentation. However, these effects only explain small amounts of colour variation (R 2 < 0.01), suggesting that historical contingency (phylogeography) may be more important than local adaptation. Temporal increases in intermediate-coloured buzzards at the expense of the extremes match the trend reported for a Dutch population, indicating widespread selection against colour extremes. Future studies should identify the mechanisms responsible for colour-related variation in fitness.

European open habitats have experienced rapid declines in bird and mammal species over the last decades, largely due to agricultural intensification. While natural predators have received considerable attention, impacts of abundant free-ranging cats (Felis catus) remain poorly studied. Diet analyses are essential for assessing population impacts, yet traditional methods often lack taxonomic resolution. DNA metabarcoding overcomes these limitations by enabling species-level prey identification. We used DNA metabarcoding of 165 faecal samples to investigate the diet of free-ranging cats in open habitats on the mainland and an island in the Netherlands. 54 taxa were detected, and 28 were identified to species-level. Based on relative read abundance, the diet consisted primarily of small mammals (53

Grassland breeding waders are in strong decline in most agricultural habitats across Western Europe. Studies evidencing the negative effects of agricultural practices on wader populations are numerous, but even in most specially managed areas the decline cannot be reversed. Earlier studies have shown that predation of nests and chicks occurs frequent, and that current predator densities can add to the decline and/or prevent recovery. In this study we experimentally study the effect of different intensities of predator control of Red Fox Vulpus vulpes, Beech Marten Martes foina, Badger Meles meles, Pine Marten Martes lotor and Raccoon Dog Nyctereutes procyonoides on nest predation and chick survival of the Black-tailed Godwit Limosa l. limosa at lake Dimmer, Lower Saxony, Germany. The area was subdivided in two subareas (Ochsenmoor and Osterfeiner Moor), between 2009-2017 343 nests were monitored. Normal predator control was implemented in both subareas in 2009 and 2010; during these years nest survival was low. From 2011 until 2017 intensified year-round predator control was implemented in Ochsenmoor and from 2016 also in Osterfeiner Moor. From 2011 until 2015 nest survival was relatively higher in the subarea with intensified predator control (Ochsenmoor). In 2016 and 2017 pre dators were intensively controlled in both subareas and nest survival was similarly high in both. From 2009 until 2017 chick survival was measured using radiotelemetry on 243 chicks. Generally, we found that in the subarea with intensified predator control, chick survival was higher. Combining the estimates of nest and chick survival, we estimate that godwit pairs raised between 0.97-1.12 fledglings per season under an intensive predator control regime and only 0.09-0.18 fledglings when predators were hunted at a normal level. As godwit pairs need to produce around 0.6 fledglings per year to sustain their population, the intensified predator control in this study-area has contributed to the recent increase in the breeding population. Our results thus show that the impact of mammalian predators on the breeding productivity of godwits can be reduced by intensified predator control. The intensified control of the targeted ground predators in this subarea could have resulted in increasing densities of unhunted predators, which subsequently could reduce nest or chick survival. Within the timespan of this experimental study, we did not observe a decrease in nest/chick survival that could hint at this meso-predator release.

Population differences in socially learned mating signals like oscine birdsong are particularly vulnerable to breakdown through dispersal.1 Despite this challenge, geographic variation in learned signals is ubiquitous.2 A proposed explanation for this pattern is that birds express predispositions to selectively learn and produce population-typical songs.3,4,5 While experimental studies on lab-reared birds have shown the existence of within-species learning predispositions,6,7,8,9,10 it remains unclear whether and how learning predispositions influence song acquisition in the wild. Here, we investigated innate song learning predispositions in wild pied flycatchers (Ficedula hypoleuca) by measuring the songs of individuals translocated as eggs from a Dutch population to a breeding population in Sweden. We compared the songs of the adult males hatched from these translocated eggs with those from the ancestral and receiving populations. Songs of translocated males closely resemble the local Swedish songs to which they were exposed during development, supporting the importance of social learning. However, translocated males selectively learned those local Swedish song elements that sound the most “Dutch-like.” As a result, their songs are significantly shifted toward those of the ancestral Dutch population. This suggests that innate learning predispositions track ongoing song evolution in wild populations of pied flycatchers. We propose that as songs continue to diverge over time, this coevolutionary relationship between song and learning predispositions may contribute to the emergence of incipient pre-mating barriers.

Conditions experienced by an individual during migration have the potential to shape migratory tactic and in turn fitness. For large birds, environmental conditions encountered during migration have been linked with survival and subsequent reproductive output, but this is less known for smaller birds, hindering our understanding of mechanisms driving population change. By combining breeding and tracking data from 62 pied flycatchers (Ficedula hypoleuca) representing two breeding populations collected over 2016-2020, we determine how variation in migration phenology and tactic among individuals affects subsequent breeding. Departure date from West African non-breeding areas to European breeding grounds was highly variable among individuals and had a strong influence on migration tactic. Early departing individuals had longer spring migrations which included longer staging duration yet arrived at breeding sites and initiated breeding earlier than later departing individuals. Individuals with longer duration spring migrations and early arrival at breeding sites had larger clutches, and for males higher fledging success. We suggest that for pied flycatchers, individual carry-over effects may act through departure phenology from West Africa, and the associated spring migration duration, to influence reproduction. While our results confirm that departure date from non-breeding areas can be associated with breeding success in migratory passerines, we identify spring staging duration as a key component of this process.

Seasonality affects the availability of resources within the African non-breeding environment of migratory songbirds. We are generally unaware of how songbirds respond to such seasonal dynamics, especially at small spatial scales that are relevant for individual birds. In this study we focus on the question of how migratory songbirds use small scale variation in seasonality in their non-breeding environment. Therefore, we measured individual movements of European Pied Flycatchers Ficedula hypoleuca in relation to habitat differences in foliation in a non-breeding site in Comoé National Park, Ivory Coast. Through a combination of remote sensing and radio tracking we show that flycatchers change their habitat use during the second half of the non-breeding season, where at the start of this period flycatchers occupy both savannah and forest, whereas with progressing foliation, after savannah burning and with the onset of the first rainfall, they narrow their site use in favour of savannah. Further measurements of arthropod abundance show that this behaviour is related to increasing numbers of particular arthropod groups during foliation, which indicates that flycatchers might track seasonal changes in food availability by moving between habitats on a small spatial scale. We hypothesize that individuals reduce their susceptibility to seasonality by establishing territories on the forest edge, where they can access both savannah and forest habitat, and thereby explore a wider variety of resources under different circumstances. In conclusion, these findings indicate that small-scale heterogeneity likely plays a key role in the ability of flycatchers to cope with seasonal dynamics on a local scale.

Abstract Climate change alters ecological interactions, and, because observed phenotypic responses are often insufficient1,2, evolutionary responses are required3. However, these have rarely been reported, and species seem to either adapt through plasticity4 or suffer population declines5. Earlier work suggested that especially long-distance migrants cannot adjust their complex annual cycle by phenotypic plasticity, because they lack predictable cues at their distant wintering grounds6,7. Here we show that the ongoing advance in spring arrival and laying dates in a pied flycatcher population results from cross-generational processes. Using a 51-year time-series we show that during the 25-year period with a strong increase in spring temperature, the population advance in laying dates could only be partly explained by phenotypic plasticity, and primarily resulted from an evolutionary change. Spring arrival from the African wintering grounds advanced simultaneously, and this was unrelated to conditions at the breeding grounds. Individual plasticity of laying date to temperature resulted from shortening the interval between arrival date and laying date, whereas the cross-generational effect was caused by advancing the arrival date. This observed evolution of migration timing can only proceed with ongoing climate change if conditions at the wintering grounds or during migration will not constrain earlier arrival in spring.

By travelling vast distances, migratory birds take advantage of earth's seasonality. Afro-Palearctic migrants can profit from lush spring conditions in temperate regions for chick rearing, but must also gain sufficient energy reserves to cross the Sahara. Rainfall during the dry season in Africa may influence the food available to birds to accumulate reserves. Conflicts of interests in resource exploitation at locations thousands of kilometres apart may occur if migrants encounter poor food conditions during these migratory preparations. Studying how wild birds adjust their fuelling and migration decisions to dynamic environments allows us to understand how flexible migrants can be, which is particularly important in an era of rapid change. We performed supplemental feeding prior to migration in individual Pied Flycatcher Ficedula hypoleuca wintering territories in Ivory Coast and remotely monitored their body mass change until they started their spring migration flight over the Sahara. We tested how access to extra food causally affects fuelling, departure mass and departure date. Seasonal fluctuations in natural arthropod availability prior to migration were monitored in two years, to explore how natural resource dynamics alters fuel accumulation. Birds that fully accessed extra food in March–April put on weight earlier and faster than birds without extra food supply, and departed 12 days earlier. Birds accumulated fuel loads that were higher than required for the Sahara-crossing, regardless of their access to extra food. Fuelling rates fluctuated in synchrony with natural conditions, as non-supplemented birds achieved the highest body mass gains at the time that natural arthropod availability peaked in the study area. Fuelling rates were lower in 2020, i.e. the year when the first rains after the dry season started late, than in 2019. Our study showed that Pied Flycatchers modulated fuelling rates – but not departure fuel loads – to food dynamics in West Africa, causing flexibility in the timing of departure. This strategy probably enhances a safe Sahara crossing, but may limit the possibilities of migrants to anticipate advancing spring conditions at breeding sites.

Palearctic migrants wintering in Africa commonly use several sites throughout the winter, a strategy known as ‘itinerancy’. In this way, migrants track spatiotemporal variation in resources. Despite the importance of this strategy for migratory landbirds, we still lack detailed understanding of how variation in environmental conditions affects site use and the timing of movements between sites. We tracked 125 adult Montagu’s Harriers Circus pygargus from Western European breeding populations between 2005 and 2018 using satellite transmitters and GPS trackers. In total, data on 129 complete wintering seasons were obtained, including 33 individuals that were followed in two or more winters. Montagu’s Harriers were itinerant, using on average 3.3 wintering sites, to which they showed high site fidelity between years. The first sites harriers used after arriving in their wintering range were situated in the northern Sahel and were dominated by natural and sparse vegetation. Subsequent sites, situated further south in the Sahel, were mainly dominated by agricultural and natural habitats. Sites used by harriers had higher habitat diversity compared to random sites. Home range size and activity (time flying per day, daily distance) peaked at the last sites harriers used (i.e. the site from which they commenced spring migration). For individuals tracked in multiple seasons, we showed that home range size did not depend on vegetation greenness. However, the birds covered longer daily distances at the same site in drier years compared to greener (wetter) years. Importantly, the timing of the movements between wintering sites was affected by local environmental conditions, with individuals staying for shorter durations and departing earlier from first sites in drier years and arriving earlier at last sites in greener years. We conclude that within the context of a strategy of itinerancy, Montagu’s Harriers are faithful to the sites they use between years (spatial component), but flexible in the timing of use of these sites (temporal component), which they adjust to annual variation in environmental conditions.

Although many recent tracking studies have uncovered considerable variation in the migratory routines of birds,1,2 the source of this variation is surprisingly poorly discussed.3 We hypothesize that a wealth of possible factors, including factors other than genetics, translate into these variable outcomes. To demonstrate how factors that are not inherited can shape migratory routine during development, we performed a translocation and delayed-release experiment with juvenile, hand-raised black-tailed godwits Limosa limosa limosa that were carefully matched for ancestral background, releasing siblings 1,000 km apart. Translocated juveniles adopted the spatiotemporal pattern of migration that is habitual for the population at the release location rather than the origin. This leads to the rejection of the hypothesis that the migration of inexperienced birds is only shaped by pre-release factors, including genes, maternal material in the eggs, and a common environment from hatching to fledging. Instead, these findings are consistent with inexperienced migrants also developing their knowledge and capacities through contextual individual learning,4 the precise nature of which now warrants study. The fact that hand-raised godwits, depending on the context in which they are released, can be induced to show different migratory routines, wintering sites, and breeding locations from their siblings indicates that processes during development offer the substrate for rapid adaptation of long-distance migration.

Many organisms fail to adjust their phenology sufficiently to climate change. Studies have concentrated on adaptive responses within localities, but little is known about how latitudinal dispersal enhances evolutionary potential. Rapid adaptation is expected if dispersers from lower latitudes have improved synchrony to northern conditions, thereby gain fitness and introduce genotypes on which selection acts. Here we provide experimental evidence that dispersal in an avian migrant enables rapid evolutionary adaptation. We translocated Dutch female pied flycatchers (Ficedula hypoleuca) and eggs to Sweden, where breeding phenology is ~15 days later. Translocated females bred earlier, and their fitness was 2.5 times higher than local Swedish flycatchers. We show that between-population variation in timing traits is highly heritable, and hence immigration of southern genotypes promotes the necessary evolutionary response. We conclude that studies on adaptation to large-scale environmental change should not just focus on plasticity and evolution based on standing genetic variation but should also include phenotype-habitat matching through dispersal as a viable route to adjust.

In most animal species, dispersing individuals possess phenotypic attributes that mitigate the costs of colonization and/or increase settlement success in new areas (dispersal syndromes). This phenotypic integration likely affects population dynamics and the direction of selection, but data are lacking for natural populations. Using an approach that combines population dynamics, quantitative genetics, and phenotypic selection analyses, we reveal the existence of dispersal syndromes in a pied flycatcher (Ficedula hypoleuca) population in the Netherlands: immigrants were larger, tended to have darker plumage, bred earlier, and produced larger clutches than local recruits, and some of these traits were genetically correlated. Over time, the phenotypic profile of the population gradually changed: each generation advanced arrival and breeding and exhibited longer wings as a result of direct and indirect selection on these correlated traits. Although phenotypic attributes of immigrants were favored by selection during the early phase of colonization, observed phenotypic changes were similar for immigrants and local recruits. We propose that immigrants facilitated initial population establishment but that temporal changes likely resulted from climate change–induced large-scale selection. This study highlights that newly established populations are of nonrandom composition and that phenotypic architecture affects evolutionary population trajectories.

In sexually size-dimorphic bird species, rearing costs of sons and daughters usually differ and may be important in the evolution of offspring sex ratio adjustment. Raptors have reversed sexual size dimorphism and the smaller males are sometimes found to be overrepresented in food-poor territories or years. As a raptor with small reversed sexual dimorphism (6% in body mass), the European Honey Buzzard Pernis apivorus is expected to show little or no brood sex ratio bias in relation to environmental conditions. We molecularly sexed 311 chicks of 195 broods in and around The Netherlands, during 1996–2014. We examined which environmental factors explained brood sex ratio variation best. Overall, sex ratio was not biased (all nests pooled: 50.8% females) but more females were produced in years when on average Honey Buzzards bred earlier (32% sex ratio change over a ten-day range in annual mean laying date). Within-year laying date variation, hatching order, abundance of wasp (Vespinae) nests (main food source) and summer weather did not explain sex ratio variation. In the Veluwe and Drenthe (1974–2014), Honey Buzzards laid eggs earlier when the spring was warmer, which resulted in a c. 9-day advance in laying date over 40 years. As warm spring weather was also a predictor of a higher density of wasp colonies, we expected female chicks to benefit more from growing up in wasp-rich years than males, if the sex ratio biases were adaptive. However, this differential growth benefit was not noticeable in chick body mass; chick body mass was best explained by negative effects of relative laying date (within a year) and hatching order. The potential benefit for female nestlings (compared to males) of growing up in years with warm springs, when egg laying occurs early and wasp colonies are more abundant, remains unknown.

Climate change has been driving long-distance migratory birds to alter their schedules under the threat of being mismatched with their food peak at the breeding grounds. It is important to study the relative contribution of environmental, genetic and ontogenetic components in various spring timing traits in the wild in order to predict the true potential for migratory birds to adapt to the changing environment. We aimed to detect if heritable and ontogenetic components can explain variation in the timing of spring migration and breeding in pied flycatchers (Ficedula hypoleuca). Geolocator tracks of 44 locally hatched birds deployed during 2016-2019 in the Netherlands and the United Kingdom were used to investigate the role of early-life traits in the pre-fledging phase, as well as parental timings, in contributing to individual differences in the timing of spring migration and breeding in adulthood. We found a positive relationship between an individuals’ birth date and spring departure date from Africa in adulthood, but not for breeding arrival or laying date. Variation in spring departure date could not be explained by any other early-life trait in the pre-fledging phase, yet was well explained by the arrival dates of its parents in its birth year. This suggests that under natural conditions, individual differences in spring departure timing have a strong heritable component (in the broad sense), but that environmental conditions experienced along the migratory route and at breeding sites are partially masking this expression in arrival and laying schedules in these early breeding populations. Such environmental masking may reduce heritability in the timing of arrival and laying, thereby slowing down climatic adaptation towards earlier time schedules in pied flycatchers.

Longitudinal tracking studies have revealed consistent differences in the migration patterns of individuals from the same populations. The sources or processes causing this individual variation are largely unresolved. As a result, it is mostly unknown how much, how fast and when animals can adjust their migrations to changing environments. We studied the ontogeny of migration in a long-distance migratory shorebird, the black-tailed godwit Limosa limosa limosa, a species known to exhibit marked individuality in the migratory routines of adults. By observing how and when these individual differences arise, we aimed to elucidate whether individual differences in migratory behaviour are inherited or emerge as a result of developmental plasticity. We simultaneously tracked juvenile and adult godwits from the same breeding area on their south- and northward migrations. To determine how and when individual differences begin to arise, we related juvenile migration routes, timing and mortality rates to hatch date and hatch year. Then, we compared adult and juvenile migration patterns to identify potential age-dependent differences. In juveniles, the timing of their first southward departure was related to hatch date. However, their subsequent migration routes, orientation, destination, migratory duration and likelihood of mortality were unrelated to the year or timing of migration, or their sex. Juveniles left the Netherlands after all tracked adults. They then flew non-stop to West Africa more often and incurred higher mortality rates than adults. Some juveniles also took routes and visited stopover sites far outside the well-documented adult migratory corridor. Such juveniles, however, were not more likely to die. We found that juveniles exhibited different migratory patterns than adults, but no evidence that these behaviours are under natural selection. We thus eliminate the possibility that the individual differences observed among adult godwits are present at hatch or during their first migration. This adds to the mounting evidence that animals possess the developmental plasticity to change their migration later in life in response to environmental conditions as those conditions are experienced.

Abstract Ecological research is often hampered by the inability to quantify animal diets. Diet composition can be tracked through DNA metabarcoding of fecal samples, but whether (complex) diets can be quantitatively determined with metabarcoding is still debated and needs validation using free‐living animals. This study validates that DNA metabarcoding of feces can retrieve actual ingested taxa, and most importantly, that read numbers retrieved from sequencing can also be used to quantify the relative biomass of dietary taxa. Validation was done with the hole‐nesting insectivorous Pied Flycatcher whose diet was quantified using camera footage. Size‐adjusted counts of food items delivered to nestlings were used as a proxy for provided biomass of prey orders and families, and subsequently, nestling feces were assessed through DNA metabarcoding. To explore potential effects of digestion, gizzard and lower intestine samples of freshly collected birds were subjected to DNA metabarcoding. For metabarcoding with Cytochrome Oxidase subunit I (COI), we modified published invertebrate COI primers LCO1490 and HCO1777, which reduced host reads to 0.03%, and amplified Arachnida DNA without significant changing the recovery of other arthropod taxa. DNA metabarcoding retrieved all commonly camera‐recorded taxa. Overall, and in each replicate year (N = 3), the relative scaled biomass of prey taxa and COI read numbers correlated at R = .85 (95CI:0.68–0.94) at order level and at R = .75 (CI:0.67–0.82) at family level. Similarity in arthropod community composition between gizzard and intestines suggested limited digestive bias. This DNA metabarcoding validation demonstrates that quantitative analyses of arthropod diet is possible. We discuss the ecological applications for insectivorous birds.

Evidence accumulates that dispersal is correlated with individual behavioural phenotype (dispersal syndrome). The evolutionary causes and consequences of such covariation depend on the degree of plasticity versus inheritance of the traits, which requires challenging experiments to implement in mobile organisms. Here, we combine a forced dispersal experiment, natural colonization and longitudinal data to establish if dispersal and aggression levels are integrated and to test their adaptive nature in pied flycatchers (Ficedula hypoleuca). We found that (forced) dispersers behaved more aggressively in their first breeding year after dispersal and decreased their aggression in following years. Strength of dispersal syndrome and direction of fecundity selection on aggression in newly colonized areas varied between years. We propose that the net benefits of aggression for dispersers increase under harsh conditions (e.g. low food abundance). This hypothesis now warrants further testing. Overall, this study provides unprecedented experimental evidence that dispersal syndromes can be remodelled via adaptive plasticity depending on the individuals' local breeding experience and/or year-specific ecological conditions. It highlights the importance of individual behavioural variation in population dynamics.

Abstract Many organisms reproduce in seasonal environments, where selection on timing of reproduction is particularly strong as consumers need to synchronize reproduction with the peaked occurrence of their food. When a consumer species changes its phenology at a slower rate than its resources, this may induce a trophic mismatch, that is, offspring growing up after the peak in food availability, potentially leading to reductions in growth and survival. However, there is large variation in the degree of trophic mismatches as well as in its effects on reproductive output. Here, we explore the potential causes for variation in the strength of trophic mismatches in published studies of birds. Specifically, we ask whether the changes in the degree of mismatch that have occurred over time can be explained by a bird's (a) breeding latitude, (b) migration distance, and/or (c) life‐history traits. We found that none of these three factors explain changes in the degree of mismatch over time. Nevertheless, food phenology did advance faster at more northerly latitudes, while shifts in bird phenology did not show a trend with latitude. We argue that the lack of support in our results is attributable to the large variation in the metrics used to describe timing of food availability. We propose a pathway to improve the quantification of trophic mismatches, guided by a more rigorous understanding of links between consumers and their resources.

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. Further, 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-terms 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 a year of the establishment, SPI-Birds counts 120 members working on more than 80 populations, with data concerning breeding attempts of almost a million individual birds over a 1700 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 of Findable, Accessible, Interoperable, and Reusable (FAIR), and aligned with the existing metadata languages (e.g. ecological meta-data language). The encouraging community involvement stems from SPI-Bird's decentralized approach: research groups retain full control over data use and their way of data management, while SPI-Birds creates tailored pipelines to convert each unique data format into a standard format. We outline the lessons learned, so that other communities (e.g. those working on other taxa) can adapt our successful model. Creating community-specific hubs (such as ours, COMADRE for animal demography, etc.) will aid much-needed large-scale ecological data integration.