Studies modelling heat transfer of bird plumage design suggest that insulative properties can be attributed to the density and structure of the downy layer, whereas waterproofing is the result of the outer layer, comprised of contour feathers. In this study, we test how habitat and thermal condition affect feather mass and density of body feathers (contour, semiplume and downy feathers) measured on the ventral and dorsal sides of the body, using a phylogenetic comparative analysis of 152 bird species. Our results demonstrate that feather mass and the density of downy feathers are higher in species that inhabit colder environments, whereas total feather density is higher of species breeding under intermediate temperatures compared to the ones breeding under more extreme conditions. The density of contour feathers, depending on the body region, is either quadratically related or negatively correlated with minimum winter temperature. The density of contour and downy feathers, measured on both sides of the body, is higher in aquatic than in terrestrial birds. However, among the former, diving behaviour does not select for further increases in body feather mass or density. The results of this study provides key insights into how the plumage of birds is adapted to different environments and lifestyles and provides a basis for understanding the diverse range and the evolution of variation in these characteristics. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13019/suppinfo is available for this article.

The mechanisms that underpin the evolution of ageing and life histories remain elusive. Oxidative stress, which results in accumulated cellular damages, is one of the mechanisms suggested to play a role. In this paper, we set out to test the “oxidative stress theory of ageing” and the “oxidative stress hypothesis of life histories” using a comprehensive phylogenetic comparison based on an unprecedented dataset of oxidative physiology in 88 free‐living bird species. We show for the first time that bird species with longer lifespan have higher non‐enzymatic antioxidant capacity and suffer less oxidative damage to their lipids. We also found that bird species featuring a faster pace‐of‐life either have lower non‐enzymatic antioxidant capacity or are exposed to higher levels of oxidative damage, while adult annual mortality does not relate to oxidative state. These results reinforce the role of oxidative stress in the evolution of lifespan and also corroborate the role of oxidative state in the evolution of life histories among free‐living birds. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13228/suppinfo is available for this article.

Sex‐specific physiology is commonly reported in animals, often indicating lower immune indices and higher oxidative stress in males than in females. Sexual selection is argued to explain these differences, but empirical evidence is limited. Here, we explore sex differences in immunity, oxidative physiology and packed cell volume of wild, adult, breeding birds (97 species, 1997 individuals, 14 230 physiological measurements). We show that higher female immune indices are most common across birds (when bias is present), but oxidative physiology shows no general sex‐bias and packed cell volume is generally male‐biased. In contrast with predictions based on sexual selection, male‐biased sexual size dimorphism is associated with male‐biased immune measures. Sexual dichromatism, mating system and parental roles had no effect on sex‐specificity in physiology. Importantly, female‐biased immunity remained after accounting for sexual selection indices. We conclude that cross‐species differences in physiological sex‐bias are largely unrelated to sexual selection and alternative explanations should be explored.

Physiological state is an emergent property of the interactions among physiological systems within an intricate network. Understanding the connections within this network is one of the goals in physiological ecology. Here, we studied the relationship between body condition, two neuroendocrine hormones (corticosterone and insulin-like growth factor 1 [IGF-1]) as physiological regulators, and two physiological systems related to resource metabolism (glucose) and oxidative balance (malondialdehyde). We measured these traits under baseline and stress-induced conditions in free-living house sparrows (Passer domesticus). We used path analysis to analyze different scenarios about the structure of the physiological network. Our data were most consistent with a model in which corticosterone was the major regulator under baseline conditions. This model shows that individuals in better condition have lower corticosterone levels; corticosterone and IGF-1 levels are positively associated; and oxidative damage is higher when levels of corticosterone, IGF-1, and glucose are elevated. After exposure to acute stress, these relationships were considerably reorganized. In response to acute stress, birds increased their corticosterone and glucose levels and decreased their IGF-1 levels. However, individuals in better condition increased their cor-ticosterone levels more and better maintained their IGF-1 levels in response to acute stress. The acute stress-induced changes in corticosterone and IGF-1 levels were associated with an increase in glucose levels, which in turn was associated with a decrease in oxidative damage. We urge ecophysiologists to focus more on physiological networks, as the relationships between physiological traits are complex and dynamic during the organismal stress response.

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