In consideration of current global climate change, ecophysiological research on wild birds has increased its emphasis on approaches related to thermal tolerance. Many studies have investigated how desert specialists are adapted physiologically to the hot and xeric conditions they live in. Our aim was to test whether migratory passerines from temperate areas also have physiological adaptations to cope with heat stress and whether such adaptations may be related to habitat or migration distance. Using video recording and flow-through respirometry, we measured temperatures of panting onset (TPANT) of 113 individuals of 14 different species, exposed to increasing ambient temperature. Our study species differed in size, migration type (short-distance migrants vs. trans-Saharan migrants) and habitat preferences (woodland, farmland, reeds). We found that trans-Saharan migrants started panting at higher ambient temperatures (TA) than short-distance migrants of similar size, but no difference between species from different habitats. This finding suggests that migrants facing a desert crossing may have adaptations to decrease the risk of dehydration while maintaining body temperature below the critical range. According to this, we suggest that there may be selection on traits related to the modulation of respiratory water loss in birds that cross the Sahara Desert during migration. Flexibility in these traits will be of crucial importance in a warmer future.

Preen oil – the secretion from the uropygial gland of birds – may have diverse functions in avian reproduction: protection against eggshell bacteria, olfactory crypsis against nest predators and olfactory mate choice. To investigate such functions, we should first characterise variation in preen oil composition, but also confirm that previously described patterns are robust. Replication studies are crucial to test the reproducibility of previous findings, but are rarely undertaken in chemical ecology. Here, we conducted an almost exact replication of a previous study on the chemical composition of preen oil in a wild passerine bird, the Pied Flycatcher (Ficedula hypoleuca). We aimed to estimate the reproducibility of the previous results using larger sample sizes and following a pre-registered analysis. In addition, we explored the ontogeny of preen oil composition by comparing nestling and adult preen oil. In line with previous findings, preen oil composition was similar between breeding partners and not repeatable within individual females across breeding stages. Female preen oil changed across breeding stages more clearly than in the original study (higher richness, diversity and volatility during incubation than nestling-rearing), further refuting a role of preen oil in olfactory crypsis in this species. Unlike the original study, we found no difference in chemical profiles between sexes (nestling-rearing), casting doubt on the proposed role of preen oil as a sex semiochemical in this species. Nestling preen oil differed from adults, was more similar to adult males than to adult females, but was not more similar to parents than to non-parents. We found family chemical signatures, which, along with the breeding pair signature, suggests an influence of the nest environment on preen oil composition. Our study highlights the importance of replication and provides novel insights into the function and development of preen oil.

Flight can be highly-energy demanding, but its efficiency depends largely on flight style, wing shape and loading, and a range of morphological and lifestyle adaptations that can modify the cost of sustained flight. Such behavioural and morphological adaptations can also influence the physiological costs associated with migration. For instance, during intense flight and catabolism of reserves, lipid damage induced by pro-oxidants increases, and to keep oxidative physiological homeostasis under control, the antioxidant machinery is upregulated. Studies on the oxidative physiology of endurance flight have produced contradictory results, making generalization difficult, especially because multispecies studies are missing. Therefore, to explore the oxidative cost of flight and migration, we explored the association between three measures of the antioxidant capacity (total antioxidant status, uric acid and glutathione concentration) and one measure of oxidative damage of lipids (malondialdehyde) with variables reflecting flight energetics (year-round or specifically during migration) across 113 European bird species using a phylogenetic framework. We found that none of the traits predicting year-round flight energy expenditure, including flight style, wing morphology and flight muscle morphology explained any measures of oxidative state measured during the energy demanding breeding period, suggesting that birds endure their everyday exercise without or low oxidative cost. However, oxidative damage to lipids and one component of the endogenous antioxidant system (uric acid), measured after the end of spring migration on breeding adult birds, increased with migration distance. Our results suggest that migration might have oxidative consequences that are carried over to subsequent life history stages (breeding).

Ticks are blood-feeding parasites which act as major vectors for various pathogenic microorganisms affecting both animal and human health. Hard ticks are known to “hitchhike” on migratory birds as they transit from breeding to overwintering grounds in spring and autumn, potentially leading to exchange and establishment of non-endemic tick species in novel environments. That said, it is not yet clear which migratory bird taxa play a role in movement of specific tick genera and what influence migratory season may have. To fill this knowledge gap, we performed a systematic literature review regarding primary data of ticks moving on migratory birds within the African-Western Palearctic flyways. In total, 34 studies were found which showed 123 bird species from 37 families connected to potential movement of 26 tick species representing six genera (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Rhipicephalus). Statistical analysis showed bird families (Ixodes: Turdidae, Prunellidae; Hyalomma: Turdidae, Strigidae, Muscicapidae, Motacillidae, Emerizidae) which carried above average number of ticks were only found for interactions with Hyalomma and Ixodes ticks with below average estimates found for all tick genera besides Rhipicephalus. Contrary to expectation, no tick genus, which was found in both migratory seasons, was estimated to have increased numbers in one season or the other. In certain cases, tick genera (e.g., Amblyomma) were only found on birds during spring migration. This pattern could highlight that the assemblage of ticks present on a bird at capture does not represent the ticks present at the point of migratory departure, highlighting an understudied importance of stopover sites to potential tick introduction or turnover. Taken together, the results presented here provide guiding information for further analysis into species specific interactions which will allow for the integration of individual level variation into understanding the risk of tick movement with migratory birds and potential for emergent disease.

Physiological stress parameters have the potential to serve as valuable early warning indicators for the conservation of animal populations. However, measuring stress in wildlife is often challenging, due to the lack of knowledge about baseline levels, and intrinsic differences between individuals across species. This study is aimed at filling this gap by investigating the influence of intrinsic factors, including sex, age, body condition, and reproductive status on the physiological stress of a forest bird community. For measuring stress levels, we used the heterophil to lymphocyte (H/L) ratio of the bird community which was assessed using a novel deep learning approach based on Convolutional Neural Networks applied to whole blood smear scans. Using phylogenetically controlled analyses across the bird species, we found higher H/L ratios in adult birds than in juveniles and observed higher stress levels in females than in males. While body condition had no effect on the H/L ratio, reproductive birds tended to have higher H/L ratios than non-reproductive birds, regardless of their sex. Furthermore, we found a robust phylogenetic signal of the H/L ratio in the studied bird community. Our results emphasize the importance of considering intrinsic factors when using stress physiology for assessing the condition of bird populations and communities.

 The Central Valley of California (CVC) and Mid-Atlantic (MA) in the U.S. are both critical sites for nationwide food security (California Poultry Federation 2016, Prosser et al. 2017), and many waterfowl species annually, especially during the winter, providing feeding and roosting locations for a variety of species. Mapping waterfowl distributions, using NEXRAD, may aid in the adaptive management of important waterfowl habitat and allow various government agencies to better understand the interface between wild and domestic birds and commercial agricultural practices. We used 9 years (2014–2023) of data from the US NEXRAD network to model winter waterfowl relative abundance in the CVC and MA as a function of weather, temporal period, environmental conditions, and landcover characteristics using Boosted Regression Tree modelling. We were able to quantify the variability in effect size of 28 different covariates across space and time within two geographic regions which are critical to nationwide waterfowl management and host a high density of nationally important commercial agriculture. In general, weather, geographic (distance to features), and landcover condition (wetness index) predictors had the strongest relative effect on predicting wintering waterfowl relative abundance in both regions, while effects of land cover composition were more regionally and temporally specific. Increased daily mean temperature was a major predictor of increasing relative waterfowl abundance in both regions throughout the winter. Increasing precipitation had differing effects within regions, increasing relative waterfowl abundance in the MA, while decreasing in general within the CVC. Increasing relative waterfowl abundance in the CVC are strongly tied to the flooding of the landscape and rice availability, whereas waterfowl in the MA, where water is less limiting, are generally governed by waste grain availability and emergent wetland on the landscape. Waterfowl relative abundance in the MA was generally higher nearer to the Atlantic coast and lakes, while in the CVC they were higher nearer to lakes. Our findings promote a better understanding of spatial associations of waterfowl to landscape features and may aid in conservation and biosecurity management protocols.

The Andean and Atlantic forests are separated by the open vegetation corridor, which acts as a geographic barrier. However, these forests experimented cycles of connection and isolation in the past which shaped the phylogeographic patterns of their biotas. We analyzed the evolutionary history of the Rufous-capped Antshrike (Thamnophilus ruficapillus), a species with a disjunct distribution in the Atlantic and Andean forests and therefore an appropriate model to study the effect of the open vegetation corridor and the Andes on the diversification of the Neotropical avifauna. We performed a phylogenetic/phylogeographic analysis including the five subspecies, using mitochondrial and nuclear genomic DNA, and also studied their differences in vocalizations and plumage coloration. Both the mitochondrial and nuclear DNA evidenced a marked phylogeographic structure with three differentiated lineages that diverged without gene flow in the Pleistocene (1.0-1.7 million years ago): one in the Atlantic Forest and two in the Andean forest. However, the two Andean lineages do not coincide with the two disjunct areas of distribution of the species in the Andes. Vocalizations were significantly different between most subspecies, but their pattern of differentiation was discordant with that of the nuclear and mitochondrial DNA. In fact, there is no song differentiation between the subspecies of the Atlantic Forest and that of the northwestern Bolivian Andes, even though they differ genetically and belong to different lineages. Consistently, no differences were found in plumage coloration between the subspecies of the Atlantic Forest and that of the southern Andes. Our results suggest a complex evolutionary history in this species, which differentiated both due to dispersion across the open vegetation corridor, likely during a period of connection between the Andean and Atlantic forests, and as a consequence of a geographic barrier in northern Bolivia. In both cases Pleistocene climatic oscillations appear to have influenced the species diversification.

Farmland habitats are witnessing steep declines in biodiversity. One rapidly declining farmland species is the Ortolan Bunting. In Finland, a staggering 99% of the population has been lost during the past 30 years. Changes in the breeding habitats have been proposed as a reason for the decline, although hazards during migration and wintering may also play a role. We gathered a 19-year data set of Finnish Ortolan Buntings and studied which spatial characteristics, habitat features, and climate factors might explain the population growth rate at the singing-group level. As explanatory variables we used region, density of small-scale landscape structures, proportion of agricultural area in the landscape, diversity of crop types, proportion of bare ground, and temperature and precipitation of previous breeding season. The only region with a marginally positive growth rate was North Ostrobothnia, where the species often occupies newly established fields. High crop type diversity mitigated the declines by perhaps providing a wide array of feeding, hiding and nesting places. Bare ground benefited Ortolan Buntings by perhaps providing an easy access to food. The last Ortolan Buntings occurred in landscapes dominated by interconnected agricultural land which, we think, reflects the species’ sociability and avoidance of forested areas. We suggest that agricultural intensification and the following potential reduction in food availability may be a cause of the decline of Ortolan Bunting. As general conservation measures, such as promoting set-aside land or field margins, have been inadequate, either in effect or in extent of application, it is evident that work remains. Northern populations of Ortolan Bunting should be targeted for further studies on feeding and breeding ecology as well as for urgent conservation actions, such as increasing crop type diversity and bare ground. Promoting more multi-functional and agro-ecologically managed agricultural landscapes would benefit a wider range of farmland species as well.

Many subcomponents of human language can be found within the vocal communication systems of other species, most notably songbirds. One of the most prominent of these subcomponents is syntax. While studies of syntax are abundant, a lack of consensus on the definition of syntax has led to much debate. The goal of this review paper is to solidify the definitions of syntax and to explore the mechanisms that shape syntax formation in songbirds and other species, with a specific focus on neurobiology, genetics, dopamine, and chunking organization.

Understanding population dynamics requires estimation of demographic parameters like mortality and productivity. Because obtaining the necessary data for such parameters can be labour-intensive in the field, alternative approaches that estimate demographic parameters from existing data can be useful. High-resolution biologging data are now frequently available for large-bodied bird species, and can be used to estimate survival and productivity. We build on existing approaches to develop a new tool (‘NestTool’) that uses GPS tracking data at hourly resolution to estimate important productivity parameters such as territory acquisition, breeding propensity and breeding success. We developed NestTool with data from 258 individual red kites (Milvus milvus) from Switzerland tracked for up to 7 years. NestTool first extracts 42 movement metrics such as time within a user-specified radius, number of revisits, home range size, and distances between most frequently used day and night locations from the raw tracking data for each individual breeding season. These variables are then used in three successive random forest models to predict whether individuals exhibited home range behaviour, initiated a nesting attempt, and successfully raised fledglings. The models achieved > 95% accurate classification of home range and nesting behaviour in cross-validation data, but slightly lower (> 80%) accuracy in classifying the outcome of nesting attempts, because some individuals frequently returned to nests despite having failed. NestTool provides a graphical user interface to manually annotate those individual seasons for which model predictions fall below a user-defined threshold of certainty. When applied to tracking data from different red kite populations in Germany, NestTool yielded accurate predictions with > 80% accuracy in all parameters. NestTool is available as R package at https://github.com/Vogelwarte/NestTool and we encourage ornithologists to adapt it for different populations and species. NestTool will facilitate the more widespread estimation of demographic parameters from tracking data to inform population assessments

The question of why socially monogamous females engage in extra-pair behaviour is long-standing in evolutionary biology. Recent theoretical work has moved away from the indirect-benefits hypothesis to explain female extra-pair behaviours, instead favouring suggestions that they are the result of pleiotropic effects. That is, a trait under strong positive selection in either or both sexes are genetically linked to another, often unrelated, trait. For example, where genes beneficial to female fecundity (contributing to within-pair solicitation of her social partner) are linked with extra-pair behaviour (soliciting copulations from extra-pair males). Here, we test two predictions from this hypothesis: We test the prediction that female divorce, measured from the number of social mates within a given year, is linked with (1) the number of extra-pair males and (2) the proportion of her offspring that are extra-pair. Our results suggest that females who frequently divorce social partners are more likely to produce extra-pair offspring than those who maintain social monogamy. However, by contrast, those females do not also have a higher proportion of extra-pair offspring. The number of broods initiated was also positively correlated with extra-pair males, probably through increased opportunity for extra-pair males to sire offspring over a longer breeding season. Our results provide an empirical example of a behavioural trait, beneficial to female fecundity, that is linked with extra-pair behaviour. These empirical results support the intrasexual pleiotropy hypothesis as a driver of female extra-pair behaviour.

Moult Terminology: Envisioning an Evolutionary ApproachThe study of avian moult has been inhibited not only by its complexity but by convoluted and often conflicting terminologies that have combined to cloud the subject. Over time, two nomenclatures have emerged with differing bases of definition. The ’life-cycle’ system is used widely in European literature (e.g., Cramp 1988, Jenni & Winkler 2020) and defines moult terms based on timing relative to current-day life-history events, primarily breeding. Conversely, the Humphrey-Parkes or ’H-P’ system (Humphrey & Parkes 1959, as modified by Howell et al. 2003), is used more widely in the New World and Australia (e.g., Higgins & Davies 1996; Pyle 2008, 2022a; Howell 2010), and defines terms based on the principle of how moults have evolved along avian lineages (Pyle 2022b).Recently, Kiat (2022) proposed that moult terminology be simplified, and advocated use of the life-cycle approach as a more understandable system to those not familiar with moult terminology. We appreciate and endorse Kiat’s (2022) plea for a simpler moult terminology, and we empathize with his frustration, but we disagree with his primary conclusion. We argue that if one intends moult to be a subject for study, then a standardized terminology based on the best scientific evidence is preferable to a system that, while perhaps more widely understood among ornithologists and the general public for passerines in boreal regions, is often imprecise or ambiguous and is difficult to apply to the majority of avian taxa on a global basis (Higgins & Davies 1996, Howell et al. 2004, Johnson and Wolfe 2018, Kiat 2022, Pyle 2022b).The debate over the best moult and plumage terminology to use has persisted since the H-P system was proposed by Humphrey and Parkes (1959) over 60 years ago (e.g., Howell et al. 2003, 2004; Jenni and Winkler 2004, 2020; Kiat 2022; Pyle 2022b; and references therein). At this point, certain opinions on which system should be favoured are calcified, and we do not wish to belabour the details yet again. In our experience, however, newer students grasp the H-P system quickly, whereas older ornithologists—including ourselves—who first learned life-cycle moult terminology, often have initial difficulty envisioning the H-P system’s evolutionary approach. Our goal with this perspective is to propose and illustrate how moults may have evolved in birds, with the hope that it will assist future ornithologists to envision and appreciate the H-P system.Adhering to the evolutionary H-P approach, we propose considering the prebasic moult, and perhaps the preformative moult, as ancestral to all modern bird lineages, having evolved from reptiles (Howell & Pyle 2015, Pyle 2022b; see also Kiat et al. 2020, 2021; Figs. 1, S1). The complete or near-complete prebasic moult occurs in all current-day avian taxa and, rather than simply a replacement of feathers, appears to be part of an endogenous restoration of body tissues (Voitkevich 1966, Murphy 1996, Kuenzel 2003) that may be ancestral to all vertebrates (King 1972). Considering the prebasic moult as homologous among modern birds is thus a parsimonious hypothesis, providing a robust nomenclatural framework that can be applied to all bird moults and plumages. The preformative moult also appears to be present in most if not all modern bird lineages, and may have evolved in reptiles as body size developed quickly in the first year of life; if not, it evolved early in avian evolution as it is found in most or all basal lineages (Figs. 1, S1). Assuming homology among all modern bird taxa, the prebasic and preformative moults can provide the framework for defining all subsequently evolved moults within the H-P system (Figs. 1, S1). Additional inserted moults, including prealternate and presupplemental moults, can evolve (both appearing and disappearing) along bird lineages such that, unlike prebasic and preformative moults, they should not be considered homologous across all birds (Howell et al. 2003, Johnson and Wolfe 2018, Pyle 2022b).Once an evolutionary basis is appreciated, moult strategies become substantially easier to compare across all species and latitudes. For example, the four underlying strategies identified by Howell et al. (2003), which are defined by the number of moults that occur within the first and later moult cycles, can be provisionally placed in an evolutionary context (Fig. 1; see Supporting information file and Fig. S1 for details). Although it is quite possible that inserted moults may have disappeared without trace along some bird lineages, envisioning how these four strategies may have evolved for current taxa has the potential to help inform a greater appreciation for the adaptive causes of inserted moults (Figure S1).We contend that the evolutionary (H-P) system is more applicable on a global basis for studying avian moult than is the life-cycle system, and we thus encourage those who use the life-cycle system to also attempt visualizing an evolutionary approach to moult terminology, rather than trying to simply synonymize H-P terms with life-cycle terms. We suggest first determining the prebasic moult cycle, then whether or not inserted moults occur in the first and/or later cycles, and using an evolutionary approach to infer the correct designation of each inserted moult. Once envisioned, learned, and appreciated, the evolutionary approach proposed by Humphrey & Parkes (1959), as modified by Howell et al. (2003), is scientifically more precise, allows the recognition of all inserted moults, and is easier to apply consistently, to all taxa and by all parties interested in the study of moult in birds. We provide more detail on taking an evolutionary approach in the Supporting information file.

As a result of a warming global climate, understanding how organisms adjust their behaviour to environmental thermal conditions has become an increasingly important question in animal biology. Temperature-driven adjustments in parental care are potentially important due to their repercussions on offspring size, quality and survival. In 2015 and 2016 we monitored 70 zebra finch (Taeniopygia castanotis) breeding attempts in the wild. We recorded the frequency of parental visits to the nest together with mean maximum ambient temperature experienced between day 7 and 14 of the nestling period. We found that for each increase of 1 °C in the daytime temperature there was a 1% reduction in the hourly rate of parental visits. Our data suggest that nestlings may receive less food under thermally challenging conditions, which is consistent with recent studies that demonstrate offspring are smaller when reared during periods of high temperature. Understanding the behavioural drivers that may contribute to the production of smaller offspring in the heat could prove useful to forecast long-term consequences for fitness triggered by climate change.

The Nanling Mountains are an important mountain range and watershed in southcentral China. Because of the abundance in relictual plant species and geological stability throughout the Cenozoic, the mountains are considered a “museum” of subtropical biological diversity. With respect to birds, however, the roles of the Nanling Mountains in impeding the dispersal of the subtropical birds and, as a result, shaping their population and community structures have received little consideration. To examine these roles, we compiled and analyzed two datasets. (1) To test the mountains’ influence on gene flow, we undertook a population phylogeographic study comparing mitochondrial COI and Cytb DNA sequences of 5 sylvioid resident bird species of the mountains (Huet’s Fulvetta Alcippe hueti, Red-billed Leiothrix Leiothrix lutea, Greater Necklaced Laughingthrush Pterorhinus pectoralis, Indochinese Yuhina Staphida torqueola, and Mountain Bulbul Ixos mcclellandii). (2) To examine differentiated community development over the history of modern birds, we examined distributional data of all species of the Nanling region using public species occurrence records. For part (1), we sampled 327 individuals from 36 sites and conducted correlation analysis of genetic and geographic distances, taking into account the landscape of the mountains. We found that the mountains do not seriously impede gene flow among populations but influenced species differently. For part (2), comparative analysis of 446 species in 81 families indicated that family membership influenced the community composition of birds in Nanling region. Variation in family distributions is attributable to both environmental and evolutionary factors. Overall, we found that the Nanling Mountains are not currently a substantial barrier to gene flow among the species we studied but act as a corridor and refuge for these birds. However, analyses on higher ranked community data suggest the mountains acted as a barrier in older times, corresponding to the known diversification events in southeast Asian avifauna.

Bird song is crucial for attracting mates and defending territories, but different types of song or different singing behaviours may be involved in acquiring or maintaining each resource. Furthermore, male songbirds may adjust when and where they sing throughout the breeding season, depending on their breeding stage. However, such relationships remain untested in several avian taxa. Here, we studied male Bermuda White-eyed Vireos (Vireo griseus bermudianus), a passerine with two distinct song types (discrete and rambling), to test the mate attraction, territory defence, and nesting stage hypotheses. We compare song rate and song perch height among different stages of the breeding season and during the non-breeding season. We show that male vireos produce both song types during the breeding and non-breeding seasons, suggesting dual roles in mate choice and territorial defence. Singing rate did not differ between the two seasons, but, within the breeding season, males without nesting duties had significantly higher song rates than males with nesting duties. Song rate was lowest during the nestling stage, which coincided with the highest rate of nest predation. Song perch height was higher during the breeding season versus non-breeding season, among males without nesting duties compared to males with nesting duties, and when males produced discrete versus rambling songs. Our findings suggest that male vireos increase their conspicuousness to prospecting females by increasing singing rate and song perch height, and that they sing during the breeding and non-breeding seasons to defend year-round territories. Collectively, our study supports the mate attraction and territory defence hypotheses of bird song and suggests that Bermuda White-eyed Vireos adjust their singing rate in response to nest predation risk.