Avian feathers are remarkably diverse in both form and function. Coloration is among the most studied feature of feathers, but we know relatively little about melanin‐based black coloration. Despite many crows ( Corvus sp.) and other corvids having black feathers that appear uniform to human perception, their feather coloration could play an important role in social communication. We therefore tested whether the coloration of American crow Corvus brachyrhynchos feathers varied by age class and sex, two socially relevant variables. Using a visual modeling approach that accounts for the visual system of American crows, we measured the coloration of American crow feathers from museum specimens. We found that feather coloration varied by age class but not sex. Older individuals had feathers with different hue and more ultraviolet than younger crows. Discriminant function analyses correctly categorized individuals into age classes based on feather coloration with high classification success. The coloration of American crow feathers did not vary based on the time since the last molt and replacement, but did vary with the time since the specimen was collected. The visual modeling approach suggests that crows can discriminate among different feather regions. One region with particularly distinctive coloration properties was a facial mask, which could potentially function to minimize eye glare. Our results suggest that feather coloration in American crows (and potentially other seemingly monomorphic corvids) could reflect underlying qualities of those individuals that are important for social communication.

Flight plays a central role in the life histories of birds but is extremely energy demanding. Reproduction is also associated with high energy demands. We might thus expect to see differences in movement behaviour between individuals that breed successfully, those that breed unsuccessfully, and those that do not breed, assuming that individuals adapt their movements in response to the constraints imposed by breeding. To investigate how the flight behaviour of individuals changes over the breeding season depending on breeding success, we used GPS‐tracking data of 59 territorials Bonelli's eagles Aquila fasciata , from the French population, collected between 2009 and 2023. While flight behaviours (range of movement, i.e. the spatial extent explored by individuals throughout the day, and proportion of time in flight) showed some variation resulting from breeding success, especially in females, it was less affected than we expected. However, after chicks had fledged, both male and female parents spent more time in flight than non‐breeders or unsuccessful individuals. This might be explained by individuals having invested so much in breeding by the end of the breeding season that to protect their investment they increase movement to ensure the survival of their young. Such behaviour may also be necessary for successful breeders to replenish the energy reserves used to rear their chicks. These results suggest that investing in reproduction may come at a significant cost to sedentary territorial birds, impacting their flight behaviour. However, the effect of this investment may only become apparent at the end of the breeding season, due to the accumulation of costs over time.

Experimental studies in humans and laboratory species have shown that the decline of the immune system with age (immunosenescence) and the accumulation of oxidative damage to macromolecules are two key contributors to the onset and progression of the ageing process. Although laboratory models have provided important insights, the physiological basis of ageing in natural populations remains comparatively understudied, constraining our mechanistic understanding of the ageing process. The complexity of age‐related physiological changes increases further in long‐lived species, which appear to possess unique adaptations that mitigate immunosenescence and oxidative damage. However, studies investigating the underlying physiological mechanisms in long‐lived birds have yielded contrasting results. In this study, we compared four markers of oxidative status and eight immune markers between younger and older breeders of a long‐lived seabird, the Scopoli's shearwater Calonectris diomedea , to identify potential physiological signatures of ageing. Regardless of sex, older individuals exhibited higher levels of blood antioxidant enzymes, natural antibodies, and lymphocytes compared to younger birds, while levels of DNA damage and cellular effectors of innate immunity did not differ between age classes. These findings suggest that older shearwaters may upregulate antioxidant enzyme activity, possibly to cope with increased basal production of reactive oxygen species, in line with the oxidative stress theory of ageing. Alternatively, the higher antioxidant levels of older birds might reflect selective mortality of birds with reduced protection. In contrast to the oxidative status, the observed immune patterns do not support the immunosenescence hypothesis.

Long‐distance migrants face numerous challenges during their migration, with the spring migration timing being particularly important, as early arrival at the breeding grounds often enhances breeding success. Both innate and environmental factors influence migration timing in birds, and the earlier arrival of males than females is common among songbirds breeding in temperate zones. The great reed warbler Acrocephalus arundinaceus males departed from the wintering grounds 10.5 days earlier than females. Interestingly, the oldest males (≥ 4‐year‐olds) departed significantly earlier than younger adult males (2‐ and 3‐year‐olds), suggesting that accumulated experience and/or age‐related hormonal changes may underlie this pattern. No significant differences were observed among female age groups. In contrast, we found no significant age effect for the duration of spring migration in either males or females. Males arrived 9.9 days earlier than females at the breeding grounds. Similar to departure dates, the oldest males (≥ 4‐year‐olds) arrived significantly earlier than middle‐aged males (3‐year‐olds), while no such age‐dependent relationship was observed in females. In both males and females, we found positive relationships between the departure date from the wintering grounds and the arrival date at the breeding grounds with a slope of β = 0.59, showing that spring migration duration becomes shorter as the spring season progresses. In addition, for a given departure date from the wintering grounds, males arrived 3.9 days earlier than females, but no significant differences were observed between age groups either in males or females. Our findings have important implications for understanding how long‐distance migratory birds might adjust their endogenous migration programs based on individual qualities, and highlight the potential role of learning and experience in these processes. This further emphasizes how adaptations to ongoing climate change might depend not only on individual quality, but also on plasticity and the age structure of the population.

Shorebirds typically spend more time fueling and refueling during migration than they spend in actual flight, and consequently their stopover ecology has important implications for fitness and conservation. We examined spatial variation and long‐term changes in stopover duration of radio‐tagged western sandpipers Calidris mauri over three decades on the Pacific Coast of North America. Birds were monitored during 10 periods of northward migration at four key stopover regions including: San Francisco Bay, CA; Willapa Bay/Grays Harbor, WA; Vancouver Island/Fraser River Delta, BC; and Copper River Delta, AK. We used Cormack–Jolly–Seber models to estimate the daily probability of staying at a stopover site (φ) corrected for daily detection rates ( p ) in relation to annual and seasonal variation. Daily probabilities of staying were then used to calculate length of stay (LOS) in days. Mean LOS decreased at stopover sites as birds progressed northward and varied from 23.1 days at San Francisco Bay to 3.6 days at the Copper River Delta. The duration of length of stay did not show a strong temporal trend by year or decade from 1992 to 2022. Effects of covariates on LOS varied by stopover site, with locally tagged birds staying 2 to 4 days longer than birds tagged farther south, females staying longer than males, and LOS being negatively correlated with mass of the bird at the time of capture with lighter birds having a longer LOS. We suggest that western sandpipers on northward migration may be limited in their ability to reduce LOS beyond the current duration over much of northern parts of the Pacific Flyway. Covariate effects demonstrate that stopover decisions of individual birds are impacted by their energy reserves, but stable lengths of stay over three decades suggest that western sandpipers may have limited flexibility to cope with environmental change in the northern part of their migration.

Avian haemosporidian parasites are globally widespread with a broad repertoire of hosts. When infected, the host can either reduce (resistance) and/or limit the severity of parasitaemia (tolerance). Oxidative stress plays a pivotal role in the host's resistance and tolerance, as well as its detrimental endpoints. The rationale behind this paradox lies in the dual role of reactive oxygen species (ROS): they are both beneficial and detrimental for the host, while being harmful to the parasite. Thus, it is in the parasite's interest to maintain a reduced environment within the host's cell, whereas the host needs a fine‐tuned balance between generating ROS to eliminate the parasites and maintaining sufficient antioxidant levels to protect itself. This dynamic we refer to as the host–parasite oxidative arms race . Here, Eurasian siskins Spinus spinus were experimentally infected with Plasmodium ashfordi to investigate how the fundamental antioxidant system – the glutathione system – responds to infection over time compared to control birds. By combining physiological and gene expression data from both the parasite and the host at different time points, we provide evidence for this oxidative arms race . The gene expression data show that the parasite aims to eliminate ROS through its high expression of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione synthetase. In contrast, the host upregulates glutathione S‐transferases (GSTs) and glutathione peroxidases (GPX), which may result in the reduced physiological levels of glutathione seen at the end of the experiment. Although the parasite seems to win the race in terms of the oxidative state of the cell, the marked decrease in parasitaemia from day 21 (44%) to day 31 (15%) suggests that the host's strategy is sufficient to defeat the parasite. Future studies should include measures of oxidative damage to reveal whether there are any long‐term costs related to the host's strategy at different time points of infection.

Resource acquisition and allocation are central to life history theory, explaining the diversity of strategies among species as well as the distribution of events over the annual cycle. Moult is a major phase in the annual cycle of birds, but explanations for moult scheduling are heavily biased towards temperate systems with seasonal breeding patterns. Our research on a year‐round breeding tropical bird, the common bulbul Pycnonotus barbatus (bulbul), tests whether moult depends primarily on stored body reserves (capital) or on resources acquired throughout the moult period (income). Making this distinction elucidates trade‐offs between moult, and other annual cycle events, and responses to environmental change. We estimated moult start date and duration in captive bulbuls whose body condition we experimentally manipulated by feeding them fruits or invertebrates 6–3 and 3–0 months before moult, and fruits or a mixed diet during moult. We studied free‐living bulbuls as reference group. We found that moult onset is best predicted by diet‐manipulated condition just before moult, while moult duration is best predicted by diet‐manipulated condition during moult. Specifically, invertebrate‐fed bulbuls started moult 33 days later than fruit‐fed bulbuls. In addition, once invertebrate‐fed bulbuls were switched to a mixed diet, they moulted 52 days quicker than fruit‐fed bulbuls, albeit still 36 days slower than free‐living bulbuls on average. Males started moult 15 days earlier and had a more variable start of 20 days, but did not moult quicker than females on average. Our findings indicate that moult in bulbuls is both income‐ and capital‐dependent, with moult initiation determined by individual body reserves and feather growth still occurring on a fruit‐only‐diet, but is significantly improved by dietary proteins from a mixed diet and in field bulbuls. In this year‐round breeding bird, moult seasonality is maintained in the absence of breeding, but heavily influenced by foraging conditions.

Parrots represent an excellent model in which to study complex vocal communication because they meet the prerequisites of the social complexity hypothesis and, like humans, have lifelong vocal learning. Despite that, the difficulty in acquiring good field data on wild parrot vocalizations has limited such research. Here, we examine the warble duets of the yellow‐naped amazon, a critically endangered species, which are used by mated pairs when defending territories. We found that these duets are marked by a large lexicon of 36 call types, with additional variants. The calls assorted non‐randomly within the duets and are organized by syntactic rules. In addition, 31% of the call types are sex‐biased (used 75–99% of the time by one sex), and another 25% of calls are sex‐specific (100% of calls given exclusively by one sex). Overall, the duets were marked by great variability; despite being structured by syntactic rules, we observed very little overt repetition in entire duets. This research provides important evidence that large repertoires and complex vocal communication are an important part of the natural communication systems of wild parrots.

Anthropogenic impacts, such as forestry and land use practices, can deteriorate habitat quality, affecting reproductive success and leading to population decline. The boreal willow tit Poecile montanus population has dramatically declined in Finland. In contrast, great tits Parus major and blue tits Cyanistes caeruleus , both of temperate origin, have expanded their ranges and increased in numbers in northern Finland. It has been suggested that the decline in the willow tit population is mainly caused by the reduction in breeding sites and wintering areas resulting from habitat loss and degradation. We investigated whether forest management influences the reproductive success of willow tits, great tits, and blue tits, including recruitment across years, while considering other factors such as possible intra‐ and interspecific competition. We combined long‐term breeding data (1990–2020) from Oulu, Finland, with spatial data on forest management activities and analysed the data using (generalized) linear mixed‐effect models. Results varied depending on the breeding parameter, species, and forest management method. Forestry had no significant effect on clutch size. We found negative effects of thinnings on nestling survival and growth in great and blue tits, with clear‐cuttings negatively affecting blue tit nestling growth. However, recent clear‐cuttings had positive effects on willow tit nestling survival and growth. Despite these benefits, clear‐cuttings near nesting sites were associated with reduced willow tit local recruitment probability, leading to an overall negative impact on the population. These findings may arise from forest management influences on food availability; certain stages following clear‐cutting may improve the food availability for boreal willow tit nestlings, yet thinnings may reduce foraging opportunities for great tits and blue tits. While some positive effects of forestry on willow tits were recorded, they may not be enough to offset the well‐established negative impacts of forestry on the loss of optimal wintering and breeding habitats.