Here we investigate the possible link between feeding rates and poor breeding success in an Atlantic puffin (Fratercula arctica) colony on the island Skúvoy in the Faroe Islands. We compare two years of field observations from an early period, 1992 (N = 26) and 1993 (N = 47), with two years of video footage from inside the burrows from 2018 (N = 9) and 2019 (N = 13). 1993 had good breeding success, whereas it was relatively poor in the other three years. Average feedings per day varied between 7.3 (in 1992) and 9.5 (in 2018). Feeding frequency did not differ significantly between early and late years or between good and poorer years. However, the breeding season was shorter in the good year and because of this, chicks were fed fewer times in that year than in the poorer years. Our results suggest the amount or quality of food brought to chicks by parents per feeding event were more important in determining breeding success than number of feedings.

Most bird species are diurnal but drastically change their diel cycle of activity to migrate at night. Nocturnal migration has been documented using different methods (e.g., experiments, radar and radio tracking, and acoustic monitoring), but accurately quantifying the proportion of nocturnal versus diurnal flight at the individual and species levels, and understanding how this behavior evolved across the avian tree, has remained methodologically challenging.1,2,3 Such uncertainty is not only of theoretical importance but also limits our ability to mitigate conservation threats, particularly from light pollution and building collisions.4,5,6 With multi-sensor geolocators recording light, barometric pressure, and activity,7,8 we reconstructed high-resolution migratory trajectories9,10 for 411 individuals from 56 small- or medium-sized landbird species across four continents and measured the proportion of each flight that occurred during night or day. Our species-level quantification confirmed nocturnal migration as the dominant strategy among small landbirds, while also providing precise flight proportion estimates across a broad taxonomic sample and refining the classification of several species previously described as partial or facultative diurnal migrants. We found that birds initiated and ended migratory flights near civil dusk and dawn, thereby maximizing nocturnal travel. The phylogenetic signal we detected indicates that nocturnal migration is largely conserved within lineages. While nocturnal migration confers multiple advantages, the relative importance of the proposed drivers remains to be determined.11,12 Our study provides species-specific quantification of nocturnal migration, highlights tracking gaps across taxa and regions, and opens new avenues for studying the evolutionary, ecological, and sensory drivers of nocturnal migratory flights.

Birds use a variety of navigational strategies, including the geomagnetic field, especially when other cues are not available, such as under overcast or nocturnal conditions. Magnetite particles in the beak, cryptochromes in the eye, cellular ion-channel alterations, and changes in the vestibular system have been proposed to explain magnetoreception, but the exact mechanisms remain debated. Here, we used physical, morphological, functional, and genomic assays to identify the presence of superparamagnetic macrophages in the liver. We found that after macrophage depletion, pigeons flying under overcast conditions lacked their usual orientation capabilities. Orientation was unimpaired in birds without macrophages when the sun was visible, suggesting that this was their primary cue. We propose that in homing pigeons, superparamagnetic macrophages in the liver are required for finding magnetic direction.

The Bluethroat Luscinia svecica is a Palearctic passerine comprising several subspecies with distinct migratory strategies. While the movements of L. s. svecica, L. s. cyanecula and L. s. azuricollis are partially known from geolocator and ringing data, the French Atlantic coast endemic L. s. namnetum has never been tracked. To address this gap, we equipped 31 males with light-level loggers in 2016 and 2017 and 20 males with multi-sensor loggers in 2024 in the core range of the taxon (Charente-Maritime, France). Five birds were recaptured the following springs, providing the first full annual cycle tracking for this subspecies. The four birds tagged with light-level loggers broadly wintered in Western Iberian coastal regions. The individual tagged with a multi-sensor logger migrated from its breeding site to southwestern Spain in seven flights, leaving in late August, staging for one month at approx. 950 m a.s.l. in central Spain, and wintered in Andalucía. Spring migration was faster and more direct, with three nocturnal flights in early March. All flights occurred at night and reached altitudes up to 4430 m a.s.l. above terrestrial grounds. These results confirm L. s. namnetum as a short-distance migrant strongly relying on specific Iberian habitats (South and Western wetlands but also central middle elevation massifs) which therefore hold a strong importance for the conservation of this range-restricted taxon.

Migratory songbirds have an internal circannual genetic programme that controls the timing and extent of migratory flight activity, as demonstrated by experiments with birds held in cages. We used multisensor data loggers to record the timing and duration of all migratory flights during the annual cycle of 15 free-living individuals of red-backed shrikes Lanius collurio. Annual actograms unexpectedly revealed that the nocturnal migratory flights of the shrikes were organized in a highly structured way, with flights aggregated into segments that could be readily identified for all individuals, showing low variability and thus high consistency between individuals. These results suggest that the execution of migratory flights is under a high degree of control according to a rather detailed internal travelling plan for the annual migration cycle. Potentially, the control of migratory flight under natural conditions depends on a complex feedback process where external cues associated with the geographic, temporal and nutritional situation of the bird are required for the internal programme to properly regulate the successive segmental flight steps of the migratory journey. This would mean that the internal/genetic programme for control of bird migration is much more dynamic and complex than hereto assumed.

Aim To evaluate (1) whether three migratory nightjar species (Family Caprimulgidae) adhere to Bergmann's rule, (2) whether environmental factors on the breeding or wintering grounds determine body size, and (3) which mechanistic hypotheses best explain Bergmannian patterns in body size. Location North and South America; Europe and Africa. Taxon Eastern whip-poor-will (Antrostomus vociferus), Common nighthawk (Chordeiles minor) and European nightjar (Caprimulgus europaeus). Methods We used GPS tracking and morphometric data to assess competing hypotheses explaining variation in body size for each species, based on their breeding (n = 3388) and wintering (n = 189) locations. Results All three species exhibited Bergmannian patterns in body size, providing the first evidence that nightjars conform to Bergmann's rule despite adaptations to severe environmental conditions. Environmental and geographic variables at breeding sites were stronger predictors of body size than wintering-site variables. Although we found partial support for Bergmann's temperature regulation hypothesis, geographic variables, rather than specific environmental factors, emerged as the strongest predictors of body size variation. Main Conclusions Latitude and longitude correlated strongly with environmental variables and migratory distance; thus, these geographical variables likely encompass many factors that influence body size in nightjars. The present study is among the first to use tracking data from individual birds to understand how environmental pressures across the annual cycle are related to body size. Our findings highlight the critical role of geographic breeding-ground factors in shaping Bergmannian patterns, offering robust evidence to support nearly two centuries of research since Bergmann's rule was first described in 1847.

Synchronous fluctuations in species' abundance are influenced by synchrony in underlying rates of productivity and survival. However, it remains unclear how rate synchrony varies in space and time, contributes to abundance synchrony, and differs among species. Using long-term annual count (number of adults captured), adult survival and productivity (number of juveniles captured per adult) data for breeding land-birds at ringing sites across Europe, we show that synchrony is strongest and largest scale in productivity and weakest and smallest scale in counts. However, counts fluctuate more synchronously with survival than they do with productivity. These patterns hold for species which do not migrate or only migrate within Europe (European-residents) and those migrating to sub-Saharan Africa (subSaharan-migrants), but the periodicity of productivity and survival synchrony is longer in European-residents than in subSaharan-migrants. This suggests that survival and productivity synchrony may interact to weaken abundance fluctuations but are influenced by environmental drivers operating over differing timescales in European-resident and subSaharan-migrant species.

Host-parasite arms races facilitate rapid evolution and can fuel speciation. Cuculus cuckoos are deceptive egg mimics that exhibit a broad diversity of counterfeit egg phenotypes, representing host-adapted subpopulations (gentes). Genome analysis of 298 common (Cuculus canorus) and 50 oriental cuckoos (Cuculus optatus) spanning 15 egg morphs revealed that eggshell background coloration is predominantly influenced by matrilineal genetic variation. Recurrent mitochondrial mutations and an ancient W chromosome-linked translocation of an autosomal assembly factor for respiratory complex I provide a tentative link between mitochondrial function and pigment synthesis through the heme pathway. Biparentally inherited loci contribute to phenotypic variation in both species, mainly for maculation. The evolutionary tug-of-war over a sex-limited, mimetic trait integrates autosomal components with the nonrecombining, matrilineal genome without catalyzing genome-wide divergence between gentes.

The Red-spotted Bluethroat (Luscinia svecica svecica) has a large breeding range in northern Eurasia and populations are known to migrate along the Indo-European or the East Asian flyway towards wintering areas in South and South-East Asia. The migration routes of birds breeding in Western and Central Siberia, where a migratory divide is expected, remain largely unknown. In this study, we tracked a Red-spotted Bluethroat with light-level geolocation from its breeding site in Central Siberia to its wintering area in the border region between north-western India and Pakistan. The bird followed the Central Asian flyway, with possible stopovers near the Tien Shan mountains during autumn as well as north and south of the Central Asian mountain ranges and in north-eastern Kazakhstan during spring migration. Our findings suggest a potential migratory divide for Red-spotted Bluethroats east of the Yenisei River, supported by contrasting migration patterns between Central and north-eastern Siberian populations. Further tracking and genomic studies are needed to explore the origins and migratory connectivity of Bluethroats across Siberia and to locate the migratory divide.

Phenological plasticity-the ability of organisms to adjust the timing of life-history events in response to environmental variability-is the primary adaptive mechanism for many organisms to changing seasonality (e.g., earlier spring). By enabling alignment between life-history events and resource availability, it helps to maintain fitness despite changing environmental conditions. Theory predicts that phenological plasticity should vary among populations because of heterogeneity in environmental variability, and among species because of differences in life-history (e.g., migration distance) and phylogenetic constraints. However, comprehensive, multi-species, and cross-population analyses of phenological plasticity remain scarce. Here, we address this gap by using a unique, four-decade dataset from Europe-wide monitoring of common songbirds. Our approach reveals how variation in phenological plasticity is structured according to site temperature properties, both within and across species. We found that long-distance migrants generally exhibit lower plasticity than residents or short-distance migrants, highlighting a fundamental constraint tied to migration strategy. Within species, populations inhabiting sites with predictable temperature profiles showed slightly stronger plastic responses, particularly among single-brooded species and those adapted to warmer breeding conditions. Notably, populations from the fastest-warming regions demonstrated marginally greater plasticity, regardless of other ecological traits, suggesting a global tendency for increased responsiveness in rapidly changing climates. These findings confirm and extend patterns previously observed at smaller scales, offering a more nuanced understanding of how local temperature conditions drive phenological plasticity. By demonstrating that the interplay between local environmental conditions and life-history traits underpins variation in breeding phenological responses, our study refines the current framework for predicting adaptive potential across populations and species under climate change.

BACKGROUNDTo spot potential introductions of West Nile virus (WNV) into Denmark, a national surveillance programme for WNV was established in 2011. The relevance of this programme was underscored in the late 2010s, when WNV was detected in areas close to Denmark.AIMWe describe the Danish WNV surveillance programme and its findings in 2011-2023.METHODSThe surveillance programme monitors mosquitoes, which are WNV vectors, and some mammalian animals and birds, which are WNV hosts. Surveillance samples are also tested for the closely related Usutu virus (USUV), which, like WNV, is a flavivirus. During the study, WNV and USUV RNAs were sought in 62 bats (sampled in 2022-2023), 5,661 Culex sp. mosquitoes (2011-2023), 628 dead wild birds (2011-2014 and 2019-2023), and 492 live birds (2011-2012, 2022-2023). These 492 birds were from among 3,269 live long-distance migratory birds (sampled in 2011-2023) serologically tested for WNV and USUV antibodies. Additionally, 4,978 free-ranging poultry (2011-2023) and 236 horses (2011-2013) were tested serologically.RESULTSNeither WNV nor USUV RNA was detected in bats, mosquitoes, or birds, but anti-WNV specific antibodies were detected in migratory birds, one domestic chicken and one imported horse. For migratory birds, competitive ELISAs detected anti-flavivirus antibodies in 3.9% (128/3,269) of tested samples. Across 2011-2023, the annual flavivirus seroprevalence varied from 1-13%. Using virus neutralisation assays on selected samples, anti-WNV or anti-USUV specific antibodies were detected in 25 and 11 bird samples, respectively.CONCLUSIONSFindings demonstrate that the concern about virus incursion is well founded and support continued vigilance for WNV.

Many birds schedule their activity to a specific phase of the diel (24‐h) light–dark cycle. Two notable exceptions are the nocturnal migration of many otherwise diurnal songbirds and the diverse activity patterns of birds during the continuous light above the Polar Circles during summer. Assumptions about the phase relationship between migratory activity and the diel cycle can be incorrect, and the diel activity of Arctic migratory birds during the year is largely unknown. We used the snow bunting Plectrophenax nivalis to investigate whether breeding songbirds above the northern Polar (Arctic) Circle have 24‐h activity rhythms with distinct active and inactive phases across the annual cycle, and whether their migration aligns with a specific phase of their activity rhythm. We found that male snow buntings maintain a robust 24‐h activity rhythm with distinct active and inactive phases across most of the annual cycle, including during polar summer, but the robust 24‐h periodicity attenuated during vernal migration. Birds scheduled long and short flights across their diel activity rhythm. However, shorter flights most often began during the active phase, and the longest flights most often began during the transition between the active and inactive phase of the diel activity rhythm. This indicates that snow buntings can flexibly schedule their flights across their prominent diel activity rhythm, although their longest flights during migration typically overlapped with their normal inactive phase, probably corresponding to nighttime. Several open questions, however, remain about the generality of our results for other songbirds, such as: what is the phase relationship between the diel activity rhythm and flight to the diel light–dark cycle, and how do historical accounts of diel migration phase align with empirical activity data for other species?

The scale of movement associated with the migration of many intra-African bird species is still poorly known even with the increasing availability of tracking devices. In this study, we tracked the movements of Black Coucals Centropus grillii breeding in Nigeria from late July using satellite telemetry. Individuals remained on the breeding site for several months; two individuals transmitted after October and these two moved shorter distances (< 100 km) away from the breeding site in early December. One of these was tracked for a full year, moving to a site 175 km south of the breeding site in early January and returning to the breeding site in late May. The bird migrated faster during the return journey (58 km day-1) as compared to the post-breeding journey (5.9 km day-1). The overall home range (90% kernel density) during breeding was 20.4 +/- 3.3 km2 (mean +/- s.d.) and the core (50%) 5.0 +/- 1.6 km2 with no apparent clear change outside of the breeding season. Vegetation conditions at the distant site were apparently poorer although in an average year the move would have led to improved conditions. Short-distance seasonal migration of Black Coucals might be widespread in drier seasonal habitats.

Little is known on the migration routes of songbirds breeding in Siberia. We used satellite transmitters to study the migration of two White´s Thrushes Zoothera aurea captured during breeding season in the Ural Mountains, Russia. One of the devices transmitted during spring and the breeding season (May to September) in the following year from the Khangai Mountains of central Mongolia, 3000 km southeast of its former breeding season site. Based on the combination of lack of accurate locations, potentially unsuitable habitat, unusual behaviour, and unusual location outside of the known breeding range we consider it most likely that the bird died there during migration, while the transmitter continued to function. No data on migration routes or non-breeding areas were received, although the movement to Mongolia suggest a migration route east of the Qinghai-Tibetan plateau.

Most migratory birds return every year to the same breeding sites and some species show a similarly high fidelity to wintering grounds as well. Fidelity to stopover sites during migration has been much less studied and is usually found to be lower. Here, we investigate site fidelity and distance to previously visited sites throughout the annual cycle in the common cuckoo, a nocturnal trans‐Saharan migrant, based on satellite‐tracking data from repeated annual migrations of thirteen adult males. All birds (100%) returned to the same breeding grounds, with a median shortest distance of only 1 km from the locations in previous year. This was in strong contrast to a much lower and much less precise site fidelity at non‐breeding sites during the annual cycle: In only 18% of the possible cases in all non‐breeding regions combined, did the cuckoos return to within 50 km of a previously visited non‐breeding site, with no significant differences among the main staging regions (Europe in autumn, Sahel in autumn, wintering in Central Africa, West Africa in spring, Europe in spring). The shortest distance to a previously visited non‐breeding site differed among the staging regions with median shortest distances for the longest stopovers of 131 km [2;1223] (median [min;max]) in Europe, 207 km [1;2222] in Sahel in autumn and 110 km [0;628] in Central Africa. The distance to a previously visited staging site decreased with the time spent at the stopover in a previous year. Understanding the drivers of recurrence and site selection in migratory birds are important for guiding conservation efforts in this group but further studies are needed to establish whether the patterns observed in cuckoos are general among terrestrial migrants with continuous distribution of habitat.

Little is known regarding the migration routes of Siberian songbird populations. Here we provide the first geolocator tracking data for an Arctic Warbler breeding in Central Siberia and compare its movements with eight long-distance ring recoveries of this species. In autumn, the tracked individual migrated eastward to a stopover site in eastern Siberia, before migrating southward through Taiwan to its non-breeding sites in the Philippines and Indonesia. During spring migration, the bird spent at least one month at stopover sites in Mongolia, before migrating to its breeding site in June. Ring recovery data confirmed the movement between the Central Siberian breeding grounds and stopover sites in northern Mongolia.

The early life movement patterns of long-lived, large raptors, such as the white-tailed eagle (Haliaeetus albicilla), remain largely unexplored. In this study, we have tracked 22 individuals of white-tailed eagles hatched in Denmark to investigate key parameters, including survival rates, causes of death, nest site fidelity, geographical distribution, and dispersal behaviors. Our analyses are based on approximately 340,000 GPS/GSM telemetry positions from the 22 individuals. We found that survival rates were lower in the first year of life, especially among females, but increased in subsequent years. The primary causes of death were wind turbine collisions and avian influenza. Movement analyses revealed that juvenile eagles frequented nest sites in August and September, with females showing earlier exploratory flights and dispersal. Both males and females spent most of their first calendar year near their nest sites. In the second calendar year, a significant proportion of their time was spent at well-known white-tailed eagle resting areas in Denmark, as well as visiting neighboring countries. Notably, one individual set a new distance record for a Danish GPS-tagged white-tailed eagle, venturing at least 1750 km away from its nest site to visit Sweden, Finland, Russia, and Norway. Our results, indicating that juvenile white-tailed eagles continue to utilize nest sites into September, suggest an extension of the recommended time period for nest protection zones provided by BirdLife Denmark (DOF).

Arctic breeding seabirds have experienced dramatic population declines in recent decades. The population of Arctic skuas (Stercorarius parasiticus) nesting on the Faroe Islands, North Atlantic, breed near the southern extent of their breeding range and are experiencing some of the largest declines. This is thought to be caused in part by increased warming due to climate change and thus, it is becoming critical to investigate the proximate and ultimate effects of the thermal environment on parental physiology, behaviour and breeding success. Behavioural observations at an Arctic skua long-term monitoring colony were undertaken during the 2016 breeding season to determine the frequencies of thermoregulatory panting, and interrupted incubation events. Incubating Arctic skuas showed thermoregulatory behaviour at air temperatures (T-a) of 9 degree celsius, which suggested that they may be operating near their upper thermal tolerance limit. Arctic skuas spent significantly more time panting as T-a increases, wind speed decreases and sun exposure increases. This relationship was apparent even within the narrow ranges of T-a (7.5-15 degree celsius) and wind speed (0-5 ms(-1)) recorded. Incubation effort was not continuous with birds leaving the nest for up to 100% of the observation block. While we found no relationship between interrupted incubation and environmental conditions, panting was only observed in birds that were simultaneously incubating eggs. These results highlight the constraints on birds during the incubation phase of breeding, and indicate a potential maladaptive behaviour of maintaining incubation despite the increased cost of thermoregulation under warming temperatures in this species. However, the relationship between thermal stress, nest absence and demographic parameters remains unclear, highlighting the importance of longitudinal and/or high-resolution studies that focus on Arctic specialists and the interrelationships between environmental factors, nest absence rates and productivity.

ABSTRACTAimOngoing climate changes represent a major determinant of demographic processes in many organisms worldwide. Birds, and especially long‐distance migrants, are particularly sensitive to such changes. To better understand these impacts on long‐distance migrants' breeding productivity, we tested three hypotheses focused on (i) the shape of the relationships with different climate variables, including previously rarely tested quadratic responses, and on regional differences in these relationships predicted by (ii) mean climatic conditions and (iii) by the rate of climate change in respective regions ranging from Spain to Finland.LocationEurope.Time Period2004–2021.Major Taxa StudiedLong‐distance migratory passerine birds.MethodsWe calculated breeding productivity from constant effort ringing sites from 11 European countries covering 34° of latitude, and extracted temperature‐ and precipitation‐related climate variables from E‐OBS and NASA MODIS datasets. To test our hypotheses, we fitted GLMM and Bayesian meta‐analytic models.ResultsWe revealed hump‐shaped responses of productivity to temperature, growing degree‐days, green‐up onset date, and precipitation anomaly, and negative responses to intense and prolonged rains across the regions. The effects of March temperature and April growing degree‐days were more negative in cold than in warm regions, except for the region with the highest accumulated heat, whereas increasing June precipitation anomalies were associated with higher productivity in both dry and wet regions. Productivity responses to climate were unrelated to the rate of climate warming.Main ConclusionsThe influence of climate on bird productivity proved to be frequently nonlinear, as expected by ecological theory. The rate of climate change is less important than regional interannual variability in climate (which is predicted to increase), but this may change with the progression of climate change in the future. Productivity declines in long‐distance migratory songbirds are particularly expected if out‐of‐norm water excess increases in frequency or strength.