Journal of Avian Biology

During long‐distance migration, many birds experience periods of either prolonged fasting (during endurance flights) or extensive feeding (during stopovers). Despite decades of research on avian metabolism during migration, many questions have remained unanswered, as such research mainly focused on targeted metabolites and fat metabolism. Here, we examined the plasma‐metabolome of two migrating passerine species before they crossed the Sahara Desert. Birds were sampled at two sites populated by Pistacia trees bearing fat‐rich fruits and at an additional site dominated by blooming Eucalyptus trees. The blood samples were analyzed using both GC‐MS and LC‐MS, using an untargeted approach. Examination of metabolic pathways activated during stopovers indicated a crucial role for cycling glucose through the Cori and Cahill cycles in resting and recovery processes. This novel perspective, conducted on free‐ranging birds, suggests the evolution of avian insulin resistance due to factors such as endurance exercise, fasting, and a preference for fatty acid oxidation during migration, akin to cell trauma recovery. We detected significant inter‐site variations in birds' polar and lipophilic metabolic profiles. We interpret the differences in the polar metabolites to be associated with the physiological state of the birds, with birds that are considered to have landed during the night prior to capture showing different metabolic profiles compared to birds that have spent more time at the stopover site. In contrast, distinctions in the lipophilic profiles of birds were associated with variations in the primary food source that was available to them in the different sites. This study underscores the challenge of interpreting commonly used indicators for assessing migrating birds' physiological state, which was predominantly derived from lipid metabolism in complex ecological systems.

Light‐level geolocators were used to record the annual migration cycles of black terns Chlidonias niger (9 individuals, 11 journeys) and common terns Sterna hirundo (7 individuals, 11 journeys) breeding in southernmost Sweden. The black terns used two different non‐breeding (Oct–Mar) regions along the Atlantic coast of Africa, either north of the equator between Senegal and Liberia (3 individuals) or south of the equator between Gabon and northern Namibia (5 individuals). All the common terns travelled to non‐breeding quarters south of the equator, mainly along the coasts of Namibia and South Africa. One juvenile common tern was tracked during the first twenty months of its life. This bird spent its first northern winter in South Africa, after which it migrated north of the equator to spend the northern summer as a one‐year‐old non‐breeder in tropical waters off Ghana, after which it returned to South Africa for its second northern winter. This record demonstrates that one‐year‐old terns may undertake extensive intra‐African migration to distant over‐summering areas. Comparing geolocator results from Swedish and Dutch black tern populations indicate that they have similar migration habits, with a possible tendency of relatively more individuals migrating south of the equator in the more northerly Swedish population (leap‐frog migration). Comparing geolocator and ringing results among common tern populations indicates a fascinating and complex pattern of scale‐dependent geographic segregation and intermixing along the coasts of Africa.

While there is ample evidence supporting genetic control of migratory behaviour in short‐lived passerines, long‐lived social species have been assumed to rely solely on cultural inheritance of migratory routes. Evidence from experimental studies supporting this idea is scarce. We tested whether the moult migration in taiga bean geese Anser fabalis has an inherited component or whether the birds need oblique social learning (where knowledge on migration is transferred from any experienced individual to any naïve individual conspecific) to carry out this journey. In many waterfowl species, non‐breeders and failed breeders migrate to remote places for wing moult while successful breeders stay at the breeding grounds and moult with their chicks. We translocated one‐year‐old taiga bean geese before their first moult migration to sites outside of the breeding range to examine whether they display innate moult migration behaviour without experienced conspecifics or not. The birds were equipped with GPS‐transmitters and released in randomly assigned groups of two. Wild control one‐year‐old birds were released immediately after capture with other non‐breeding geese, while a procedural control group consisting of older birds was held in captivity until being released at the same time with the translocated one‐year‐old birds but in the place where they were captured. Most translocated birds found conspecifics and either joined locally moulting breeders or followed experienced birds to moulting sites in Russia. Two of the translocated birds did not find other bean geese and settled to moult together in southwest Finland. The wild control birds moult‐migrated as expected, while only one of the procedural control birds moult‐migrated to Russia and the remaining three stayed with locally moulting breeders in Finland. Our results support the idea that moult migration in geese is culturally inherited, highlighting the importance of the non‐relative, experienced adult individuals in maintaining population‐specific behaviours.

Understanding the behavior of migratory birds can help determine levels of connectivity and inform conservation actions for species of conservation concern. The common tern Sterna hirundo is a long‐distance migratory seabird that is considered a species of conservation concern in the North American Great Lakes region and that has experienced significant declines in breeding numbers across large lakes in Manitoba. To better understand the movement ecology of common terns, we used data from multiple tracking technologies (solar geolocation, GPS tracking, and Motus radio tracking) obtained from individuals (n = 83) across five breeding colonies on four inland lakes in North America. We identified key stopover regions used during southward migration and explored how demographics and social interactions influence connectivity. We identified three key stopover regions (Lake Erie, the southern Atlantic Coast, and Florida) and documented, for the first time, differences in post‐natal and post‐breeding migration for inland nesting terns. Juveniles arrived, on average, three weeks later than unrelated adults to their first major staging area. Although adult female arrival to and departure from Lake Erie was similar to adult males, female schedules became significantly earlier than males as southward migration progressed. Using a graph network to describe the spatiotemporal associations among adults from the same inland lake, individuals appeared to be highly connected, meeting up in different regions throughout the non‐breeding season, suggesting that social interactions may play an important role in maintaining spatial connectivity. Despite differences in migration schedules by sex and arrival to the first major staging area by age class, birds appeared to rely on the same key stopover regions during southward migration. The stopover regions identified in this study can help identify potential bottlenecks and guide future research aimed at assessing the impacts of climate change and human disturbance on common terns breeding in North America.

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 move passively (i.e. ‘hitchhike') on migratory birds as they transit between breeding and non‐breeding grounds. This potentially leads to exchange and establishment of non‐endemic tick species in novel environments. However, it is not yet clear if all migratory bird taxa play an equal role in movement of specific tick species, especially outside of medically important tick taxa. To clarify these interactions, we performed a systematic literature review regarding primary data of ticks moving on migratory birds within the African–Western Palearctic flyways. In total, 35 studies were found which showed 123 bird species from 37 families and 12 orders connected to potential movement of 30 tick species representing six genera (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Rhipicephalus). Most tick species did not show high abundance for any bird species, or, if they did, only on very few. Only Ixodes ricinus and Hyalomma marginatum were estimated to be carried at above average burdens by multiple bird species. This could indicate an increased likelihood of these species to be moved during migration. Specific tick species or whole genera were only found in certain migratory seasons as expected based on their geographic distributions. Even so, species found in both migratory seasons did not differ in their estimated abundances on birds between seasons. This result could suggest that tick abundance on migrating birds is not always a direct result of geographic distribution and may suggest an understudied importance of stopover sites towards potential tick introduction or turnover. Taken together, the results presented here provide guiding information for future analyses integrating individual level variation into the current understanding of tick movement with migratory birds.

Altricial songbirds transform themselves from naked poikilotherms to fully feathered endothermic homeotherms over a matter of days from hatching to fledging. The ontogeny of endothermy is a developmental milestone for birds that not only face warmer average temperatures, but also increasingly frequent cold snaps and extreme weather. The timing of development of endothermy has been studied in altricial birds for over half a century. However, the determinants and constraints of the onset of endothermy are not yet fully understood. We experimentally investigated whether brood size influences the ontogeny of endothermic heat production in 4–8 day‐old nestling blue tits Cyanistes caeruleus in southern Sweden. The thermogenic response to a cooling challenge (15°C) increased with age overall. We found that 8‐day‐old nestlings from reduced broods had a slightly increased capacity for endothermic heat production compared to enlarged broods. This difference cannot be explained by body mass because this trait did not differ between brood size categories. Although a metabolic response was present in most nestlings by day 6, it was brief, not lasting more than a few minutes, and not sufficient to maintain a stable body temperature in any age group. Our study shows that incipient endothermy is present at an early age in nestling blue tits and may advance faster in reduced broods, but that individual nestlings lack sufficient insulation and thermogenic performance to maintain homeothermy independently during the first week of life.

Temporal variation in resource availability, amplified by global change, may have strong impacts on species breeding at temperate and high latitudes that cue their reproduction to exploit seasonal resource pulses. This study examines how resource availability and parental care influence niche partitioning between and within age classes in the rufous‐collared sparrow, which provides extensive parental care. We hypothesized juveniles would exhibit narrower niches focused on high‐quality resources compared to adults, regardless of resource availability. We used stable isotope analysis to quantify individual and population niches in juveniles and adults across the breeding season in two cohorts experiencing contrasting resource landscapes. Contrary to our initial hypothesis, juveniles exhibited greater among‐individual diet variation and smaller total niche widths (i.e. higher levels of individual specialization, IS) during periods of high food availability in comparison to periods of food scarcity. Interestingly, total niche width and IS of adults remained stable across seasons despite a shift in trophic level, highlighting their potential role in providing a consistent diet for their young. These findings reveal a dynamic interplay between resource availability, parental care, and IS, with important implications for understanding population resilience under variable resource scenarios. The study also suggests that adult sparrows modify their provisioning strategies based on resources, potentially buffering offspring from environmental fluctuations. Understanding age‐specific responses to resource variation is crucial for predicting species responses to ecological conditions, particularly in regions like central Chile where seasonal resource limitation is expected to become more variable in response to climate change.

Many building blocks of human language can be found within the vocal communication systems of other species, most notably songbirds. One of the most prominent of these building blocks is syntax. While studies of syntax are abundant, a lack of consensus on the definition of syntax in non‐human animal communication studies has led to much debate. Consistent and deliberate use of terminology is needed to facilitate understanding across disciplines. In addition, new terminology may better describe syntactic structure found in vocal signals that are devoid of semantic associations, such as birdsong. Here, we propose two terms to describe the types of syntax commonly found in birdsong: sequential syntax and dialectical syntax. Sequential syntax can be defined as the rules that govern the patterns of sound without regard to semantic meaning. Dialectic syntax can be defined as sequential syntax that is distinct among different populations or groups with behavioral significance for those groups. Taken together, these two terms can describe the type of syntax seen in ornamental signals, such as birdsong.

Intra‐Africa movements of most African migratory birds remain an enigma. We describe the migrations of Wahlberg's eagle Hieraaetus wahlbergi using GPS‐GSM transmitters on adult eagles in their South African (n = 3) and Kenyan (n = 7) breeding areas between 2018 and 2022. The dataset included 57 migratory tracks, 29 post‐breeding and 28 pre‐breeding. We found long‐distance migrants (LDMs; from South Africa) and short‐distance migrants (SDMs; from Kenya) using common non‐breeding areas centered in the Sudans and Central African Republic. The timing of annual phases was similar, but LDMs departed on their pre‐breeding migration on average later than SDMs (13 August versus 31 July) and arrived later on their breeding grounds (13 September versus 10 August). Conversely, the average departure date on the post‐breeding migration was 4 April for SDM and 23 March for LDMs. LDMs spent significantly less time of the year than SDMs on breeding grounds (44 versus 57%), and slightly but not significantly more time (40 versus 38%) on non‐breeding areas. The post‐breeding migration distance was on average 3413.9 ± 170.9 km for LDMs and 491.9 ± 158.5 km for SDMs. At non‐breeding areas, LDMs reached more northerly latitudes than SDMs, increasing the pre‐breeding migration distance to 4495.9 ± 372.5 km for LDMs versus 1701.9 ± 167.3 for SDMs. Daily flight distances back to the breeding areas averaged 153.4 ± 130.3 km for LDMs and 167.4 ± 122.3 km for SDMs and to non‐breeding areas were shorter for SDMs (124.8 ± 113.0 km) than LDMs (178.0 ± 134.4 km). Migration speed was similar across populations and for pre‐ and post‐breeding migrations. LDMs used more stopover days than SDMs. We conclude that Wahlberg's eagles from different parts of Africa have adapted their migration to differences in timing of the breeding season, distance of travel, and resources in the landscapes encountered during migration.

Ground‐nesting shorebirds must balance the need for acoustic communication at the nest with the constant threat posed by predators. Although it may seem likely that their calls are adapted to minimize detection by predators, little is known about how these birds communicate at the nest or whether they employ cryptic strategies to avoid predation. Using passive acoustic devices and software to analyse extensive acoustic data, we quantified and categorised the calls of black‐tailed godwits Limosa limosa limosa recorded throughout the whole incubation at eight nests at a dairy farm in the Netherlands in March–June 2021. While incubating, godwits frequently use five main call types, with distinct diurnal patterns and high variation in the number of calls between breeding pairs. Birds used two quiet calls, one for communication at the nest and a second without an easily suggested meaning. Three loud calls were presumably used for predator alert, territory establishment, and long‐range communication. Interestingly, although nests were close to each other and exposed to the same aerial predators, the involvement of incubating birds in predator alert calling consistently differed. Furthermore, we described the relationship between the number of predator alert calls and the probability of a godwit flying off the nest. Our findings show that incubating godwits predominantly use loud vocalizations during the day, with only a few calls at night, which were more frequent on nights with a full moon. These descriptive findings for a single godwit community should now be expanded to other contexts, experimental situations, and shorebird species.

The western capercaillie Tetrao urogallus, hereafter capercaillie, is the largest galliform bird present in the boreal and montane forests of the Western Palearctic. Precise and accurate methods for estimating the number of individuals and/or their densities are crucial for the proper management of its free‐ranging populations. However, obtaining reliable estimates of the abundance of populations of wild species and, particularly, of birds is not a simple task. In the case of lek‐mating birds such as capercaillie, surveys are traditionally based on lek counts, that is, counts of calling males present in their mating areas: the leks. This study was carried out on the Pyrenees at six capercaillie leks where two different lek counting approaches were performed: hide‐based and walk‐based. The results were compared with those obtained from an estimate of minimum population size (MPE) derived from genotyping all faeces samples found in the lek area, and with a population size estimate derived from a genetic mark‐recapture model (N^o) of each capercaillie lek. The results of N^o were used to estimate the detection rate (D) of both lek count approaches. Our results show that traditional lek counts do not detect all male capercaillies since the detection rate was 0.34 (95% CI: 0.26–0.43) for hide‐ and 0.56 (95% CI: 0.43–0.68) for walk‐based lek counts. Our results suggest that the walk‐based lek counts were more efficient than the hide‐based ones, providing more accurate results compared to the N^o estimate. The combination of non‐invasive sampling with genetic mark‐recapture model was found to be the most reliable method for obtaining the N^o of leks given that traditional lek counts underestimate the number of capercaillie and, furthermore, can cause disturbance to the species at these sites.

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 frequently available for large‐bodied bird species and can be used to estimate survival and productivity. We extend existing approaches and present a freely available tool (‘NestTool') that uses GPS tracking data at hourly resolution to estimate important productivity parameters such as home range establishment, breeding initiation, and breeding success. NestTool first extracts 42 movement metrics such as time spent 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 independent random forest models to predict whether individuals exhibited home range behaviour, initiated a nesting attempt, and successfully raised fledglings. We demonstrate the use of NestTool by training models with data from 258 individual red kites Milvus milvus from Switzerland tracked for up to 7 years, and then applied those models to tracking data from different red kite populations in Germany where detailed observations of nests and their outcomes existed for validation. The models achieved > 90% accurate classification of home range and nesting behaviour in validation data, but slightly lower (80–90%) accuracy in classifying the outcome of nesting attempts, because some individuals frequently returned to nests despite having failed. NestTool provides a graphical user interface that allows users to manually annotate individual seasons for which model predictions exceed a user‐defined threshold of uncertainty. NestTool will facilitate the estimation of demographic parameters from tracking data to inform population assessments, and we encourage ornithologists to test NestTool for different species.

Flight can be highly energy demanding, but its efficiency depends largely on flight style, wing shape and wing 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 can be 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 used samples collected during the breeding season from 113 European bird species and explored the associations of measures of antioxidant capacity (total antioxidant status, uric acid and glutathione concentration) and oxidative damage of lipids (malondialdehyde) with variables reflecting flight energetics (year‐round or specifically during migration) using a phylogenetic framework. We found that none of the traits predicting year‐round flight energy expenditure (flight style, wing morphology and flight muscle morphology) explained any measures of oxidative state. Our results suggest that birds endure their everyday flight exercise without or with 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 could have oxidative consequences that might be carried over to subsequent life‐history stages (breeding).

A growing number of studies suggest that individuals can develop long‐term foraging specializations independently of phenotypic or environmental variation, yet little is known about how the foraging niche is acquired. The early learning of the foraging niche hypothesis suggests a key role of vertical cultural transmission in shaping the foraging niche of vertebrates. In birds, direct evidence from natural conditions is limited to a single study that cross‐fostered two related species. To date, no study has tested whether the diet received as an offspring determines the diet delivered as a parent within a single species. We tested the early learning of the foraging niche hypothesis using a Mediterranean population of great tits Parus major, which show great diet variability and moderate consistency in the diet they provide to their offspring across years. To do this, we recorded prey delivered to 9–14 day‐old chicks over twelve years. Then we assessed vertical transmission of dietary specialization using data (percentage of caterpillars, spiders, and other prey types, as well as mean prey size) from individuals recorded as a chick and as an adult. We standardised the data to control for environmental factors and ran a Linear Model for each prey type to measure individuals' consistency within the group (relative consistency), correlating the diet they received as a chick and the one they provided to their own chicks at the adult stage. The correlations between the diet received as a chick and the diet provided as a parent were either not significant or negative. Hence, although individuals showed relatively consistent foraging niches across years regarding their parental provisioning behaviour, these diet preferences were not correlated to the diet they received in the nest. Further research is needed to determine whether the foraging niche is acquired during the post‐fledgling stage.

Migratory birds excel in phenotypic flexibility, adapting physiologically as their life histories and environments require. Discerning the metabolic processes underlying migrants' physiology, an emergent property of multiple continuous and dynamic organism–environment interactions, is therefore challenging, particularly under natural conditions. Accordingly, analyses of snapshot‐sampled serum‐circulating metabolites, versatile and readily applicable for migrating birds, have increasingly become the method of choice for such physiologic inference. However, the atemporal nature of single sampling might obscure the links between observed metabolite concentrations and the processes producing them, necessitating an analytical decoupling of focal processes from their broader biochemical background. In the present study, we examined how variation in combined fat and muscle fuel stores, traits pivotal in migratory context, relates to the serum‐circulating metabolomes of spring‐migrating Eurasian blackcaps stopping‐over. Our analyses accounted for potential spatiotemporal influences in the form of time past night's fasting and random local conditions across three sites within the Negev Desert. We shifted the focus from compound‐level analysis of preselected metabolites towards the level of inclusive metabolome, quantifying serum‐circulating lipophilic and polar molecules via UHPLC–MS/MS untargeted metabolomic technique. Our results indicated a general relationship between fuel stores and the metabolome, comprising 16 326 lipophilic and 6923 polar compounds, among which 918 and 44 were annotated, respectively. By applying generalized latent‐variable linear modeling (GLLVM) upon concentrations of annotated metabolites, we identified several candidate biomarkers, some novel in migratory context, notably the fuel‐associated increase in serum ceramides likely derived from circulating very low‐density lipoproteins (VLDLs). Relying on estimated metabolite links with fuel and foraging time and on modeled residual covariations among metabolites, we demonstrate fuel–metabolite associations generally consistent with higher fat‐ and lower protein mobilization in birds having greater stores and with decreased fuel utilization as ingested nutrients accumulate over time, thus introducing a novel approach for the physiological study of migrating birds.

The extensive use of pesticides has been recognized as one of the major factors negatively impacting birds in agricultural habitats. One of the pesticide groups most used worldwide are triazole fungicides due to their effectiveness in controlling phytopathogenic fungi in cereals, vineyards and orchards. In the last decades, different experimental studies have reported important negative effects on the health and fitness of birds after exposure to triazoles. Birds can be exposed throughout the year through different routes, including oral uptake, dermal contact with treated surfaces and inhalation by overspray. Yet, the ingestion of treated or sprayed material is the principal route. The most alarming effect of triazoles, which can even occur several months after cessation of the exposure, is the decreasing reproductive outputs of birds, including delay in the onset of laying dates, reduced clutch size and hatching rate, and increased mortality of chicks. In order to synthesize the data and knowledge about the toxic effects of triazoles at different levels of biological organization, here we propose an dverse outcome pathway (AOP) on the mechanisms by which triazoles can affect avian reproduction and physiology. The reported effects highlight that the current risk assessment needs some improvements to avoid undesired effects on birds, especially long‐term effects that can influence stability and viability of avian populations from agricultural habitats.

The Andean and Atlantic forests are separated by the open vegetation corridor, which acts as a geographic barrier. However, these forests experienced cycles of connection and isolation in the past, which shaped the phylogeographic patterns of their biotas. We analysed the evolutionary history of the rufous‐capped antshrike Thamnophilus ruficapillus, a species with a disjunct distribution in the Atlantic and Andean forests and thus 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 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 signs of 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, we did not find 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 the effect of the Bolivian Altiplano as a geographic barrier. In both cases, Pleistocene climatic oscillations appear to have influenced the species diversification.

Female song in birds is more widespread than previously thought, and studies across different species are critical for better understanding the entire evolutionary process of bird song. In this study, we recorded the songs of males and females in a duetting, montane bird species, the Elliot's laughingthrush Trochalopteron elliotii, across consecutive breeding seasons. We specifically focused on identifying the number and structure of different song types by males and females, and compared these acoustic structures between the sexes. Our findings revealed that both males and females sang sex‐specific solos. More specifically, females sang a single type of solo that varied significantly in the number of notes, whereas males produced three different solos composed of 2, 3, or 4 notes, respectively. Female solos exhibited significantly more notes and longer song duration compared to males. Male solos typically had a significantly higher maximum frequency for the entire song. No significant differences were observed in the duration of the first note, song rate, and other frequency characteristics between male and female solos. Furthermore, paired males and females coordinated their sex‐specific solos to form duets when challenged by conspecific territory intruders, both within and outside the breeding season. Sex‐specific solos suggested that male and female songs play different roles and may be subject to different selective pressures. Further research is necessary for elucidating the functions of male song, female song, and duets in this montane bird species.

Avian nest design varies depending on environmental factors but may also be influenced by between‐species interactions. In the brood parasitism context, hosts may evolve nest architectures that may limit parasite access to the nest cup, reduce parasite laying success or hinder parasite chick success. Therefore, nest characteristics may reduce the likelihood or minimise the costs of being parasitised. The common redstart Phoenicurus phoenicurus is a regular host of the common cuckoo Cuculus canorus, for which cuckoo eggs are often laid outside the nest cup, resulting in low effective parasitism rates. This allowed us to evaluate variation in host nest design and test whether nest design characteristics correlate with brood parasitism likelihood and cuckoo laying success (i.e. cuckoo egg laid in the nest cup versus outside the nest cup). While recording brood parasitism events in two distant redstart populations, we documented nest cup characteristics, such as internal dimensions, materials used and nest cup position, along with the nest‐box dimensions. Cuckoo parasitism likelihood was lower for redstart nests in cavities with smaller entrances, for redstart nests with smaller nest cups and with nest cups that were built level to the rim material. For parasitised nests, cuckoo laying success was lower at redstart nests with nest cups placed further from the cavity entrance. Our results suggest a conditional process, where the cavity entrance size first prevents brood parasites access, then the cup size and the cup level in reference to the rim material affect the cuckoo choice, and finally, the nest cup position hinders cuckoo's laying success. The use of multiple nest design strategies may explain the current low effective parasitism rates in this system. Host nest design may serve as a frontline defence that could shape parasite's preferences, and consequently host nest characteristics.

Urbanization reflects a major form of environmental change impacting wild birds globally. Whereas urban habitats may provide increased availability of water, some food items, and reduced predation levels compared to rural, they can also present novel stressors including increased light at night, ambient noise, and reduced nutrient availability. Urbanization can also alter levels of brood parasitism, with some host species experiencing elevated levels of brood parasitism in urban areas compared to rural areas. Though the demographic and behavioral consequences of urbanization and brood parasitism have received considerable attention, their consequences for cellular‐level processes are less understood. Telomeres provide an opportunity to understand the cellular consequences of different environments as they are a well‐established metric of biological state that can be associated with residual lifespan, disease risk, and behavior, and are known to be sensitive to environmental conditions. Here we examine the relationships between urbanization, brood parasitism, and blood telomere lengths in adult and nestling song sparrows Melospiza melodia. Song sparrows are a North American songbird found in both urban and rural habitats that experience high rates of brood parasitism by brown‐headed cowbirds Molothrus ater in the urban, but not the rural, sites in our study system. Among adults and nestlings from non‐parasitized nests, we found no differences in relative telomere lengths between urban and rural habitats. However, among urban nestlings, the presence of a brood parasite in the nest was associated with significantly shorter relative telomere lengths compared to when a brood parasite was absent. Our results suggest a novel, indirect, impact of urbanization on nestling songbirds through the physiological impacts of brood parasitism.

Offspring sex ratios may deviate from parity when the fitness benefits of producing male or female offspring vary. We tested for sex ratio bias in smooth‐billed anis Crotophaga ani, a communal laying cuckoo with low within‐group relatedness and high offspring dispersal. One male group member performs nocturnal incubation and sires more offspring than other males in the group, suggesting males may have greater reproductive variance than females. We hypothesized that pre‐breeding rainfall influences food availability and offspring sex ratio, predicting that breeding females skew production towards the sex with higher reproductive variance (males) in high food years. Females may also adjust sex ratio across the hatching order to increase survival of the more competitive sex, especially when clutches are larger and within‐brood competition is higher. As adults, male smooth‐billed anis are larger than females, so we assumed male nestlings are more competitive than females and predicted a male‐bias in first hatched chicks in larger broods. Contrary to our first prediction, offspring sex ratio was male biased when pre‐breeding rainfall was lower. In partial support of our second prediction, marginally more first hatched chicks were male in larger broods. To our knowledge, this is the first evidence of offspring sex ratio bias in a communal laying bird species. Future work in this system will attempt to uncover the mechanisms by which co‐breeding females adjust offspring sex ratio and test alternative hypotheses to explain male‐biased offspring sex ratios under different conditions.

Rising temperatures and anthropogenic noise are two of the most pervasive and well researched anthropogenic stressors affecting avian species globally. Despite often triggering similar behavioural responses in birds, and frequently co‐occurring (particularly in urban areas), the impact of these stressors are primarily investigated in isolation. Here, we discuss and compare the most commonly researched effects of anthropogenic noise and rising temperatures on avian behaviour. We then outline recent findings on the impacts of these two stressors on cognition in birds, which underpins many behavioural adjustments. We find that both anthropogenic noise and high temperatures, when investigated in isolation, impact avian behaviours such as foraging, the antipredator response, and interactions with conspecifics. We also find that both these stressors can lead to cognitive impairment, but the occurrence and magnitude of impairment varies depending on the cognitive trait examined. Finally, we discuss the limited studies that have investigated these two anthropogenic stressors simultaneously and outline different scenarios in which additive, synergistic, or antagonistic effects of these stressors may occur. We hope our review will stimulate researchers to investigate the simultaneous effects of these and other anthropogenic stressors on the behaviour and cognition of urban‐living wild birds.

Repetition of the same vocal stimulus during vocal learning may result in habituation. Therefore, selection may favor vocal tutors that produce vocal stimuli with characteristics that reduce the risk of habituation. Superb fairywren Malurus cyaneus mothers produce a two‐element (A and signature B) call to embryos, but embryos only produce one element type (B) after hatching. Why do mothers use the A element to embryos? We broadcast calls with one (B) or two (AB) element types and measured embryo response. Embryos habituated to calls with one element type and remained responsive to calls with both. We conclude that signal characteristics in tutors may guide learning to retain pupil attention during learning.

The likelihood of a new migratory route emerging is presumably a function of 1) the associated fitness payoff and 2) the probability that the route arises in the first place. It has been suggested that diametrically opposed ‘reverse' migratory trajectories might be surprisingly common and, if such routes were heritable, it follows that they could underlie the rapid evolution of divergent migratory trajectories. Here, we used Eurasian blackcap (Sylvia atricapilla; ‘blackcap') ringing recoveries and geolocator trajectories to investigate whether a recently evolved northwards autumn migratory route – and accompanying rapid northerly wintering range expansion – could be explained by the reversal of each individual's population‐specific traditional southwards migratory direction. We found that northwards autumn migrants were recovered closer to the sites specified by an axis reversal than would be expected by chance, consistent with the rapid evolution of new migratory routes via bi‐axial variation in orientation. We suggest that the surprisingly high probability of axis reversal might explain why birds expand their wintering ranges rapidly and divergently, and propose that understanding how migratory direction is encoded is crucial when characterising the genetic component underlying migration.