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Feathers have a diversity of functions in birds and are costly to produce, so their growth rate and mass can be reliable indicators of nutritional condition at the time of production. Despite the potential for feather metrics to advance our understanding of foraging, they are underused in avian ecology. One reason for this is the difficulty of interpreting whether individual variation is driven by ontogenetic, genetic or environmental effects, which is exacerbated by the fact that most analyses have been done on cross‐sectional data. We addressed this deficit using a longitudinal dataset of tail feathers collected from great tits Parus major to test for ontogenetic and genetic effects on growth rate, mass and length, while controlling for body/feather size differences and other confounding factors. First, we found that the type of moult episode and experimentally‐induced replacement differentially affected the length, mass and growth of feathers, providing evidence of an ontogenetic effect that should be considered when comparing these feather traits across individuals as a measure of condition. Second, we detected moderate to high repeatability and heritability values from parent–offspring regression for these three feather traits, which are suggestive of an underlying genetic component of variation. Third, we used a mean centring within‐individual approach to test whether feather growth rate and feather mass (length‐corrected) are indeed positively correlated with each other as overlapping indicators of body condition in birds, and found that this association, although positive, is weak and only significant between individuals. This suggests that both metrics are not so intimately linked as originally thought, and probably have different sensitivities to variation in foraging performance and ecological conditions. Together with the higher plasticity of feather growth rate compared to feather mass, our results support the idea that feather growth rate is better suited for examining short‐term responses to environmental variation. [ABSTRACT FROM AUTHOR]

Feathers have a diversity of functions in birds and are costly to produce, so their growth rate and mass can be reliable indicators of nutritional condition at the time of production. Despite the potential for feather metrics to advance our understanding of foraging, they are underused in avian ecology. One reason for this is the difficulty of interpreting whether individual variation is driven by ontogenetic, genetic or environmental effects, which is exacerbated by the fact that most analyses have been done on cross‐sectional data. We addressed this deficit using a longitudinal dataset of tail feathers collected from great tits Parus major to test for ontogenetic and genetic effects on growth rate, mass and length, while controlling for body/feather size differences and other confounding factors. First, we found that the type of moult episode and experimentally‐induced replacement differentially affected the length, mass and growth of feathers, providing evidence of an ontogenetic effect that should be considered when comparing these feather traits across individuals as a measure of condition. Second, we detected moderate to high repeatability and heritability values from parent–offspring regression for these three feather traits, which are suggestive of an underlying genetic component of variation. Third, we used a mean centring within‐individual approach to test whether feather growth rate and feather mass (length‐corrected) are indeed positively correlated with each other as overlapping indicators of body condition in birds, and found that this association, although positive, is weak and only significant between individuals. This suggests that both metrics are not so intimately linked as originally thought, and probably have different sensitivities to variation in foraging performance and ecological conditions. Together with the higher plasticity of feather growth rate compared to feather mass, our results support the idea that feather growth rate is better suited for examining short‐term responses to environmental variation.

Moult of birds is shaped by environmental and genetic drivers whose relative contribution to the structure of feathers may differ within and between populations. In this study, we compare some traits of tail feathers (growth bars, mass, rachis width and barb length) between four populations of the Eurasian blackcap Sylvia atricapilla breeding at different elevations within the southwestern Palaearctic. We tested if these traits were related to the primary productivity of habitats (a surrogate of food availability) or were better explained as an adaptation to altitudinal movements. The distribution of primary productivity was positively related to blackcap abundance suggesting that the species tracked the most productive areas to breed. In this environmental setting, wing morphology (wing length, concavity and pointedness) suggested that lowland blackcaps were sedentary while blackcaps from highland areas were involved in altitudinal movements. The feathers of blackcaps inhabiting the highlands showed wider growth bars and rachis than those of the most productive lowland areas but did not differ in feather mass and barb length. Fast feather growth has been related to time constraints to moult and wider rachis to improve flight efficiency in migratory birds. Our results therefore suggest that differences in feather characteristics between southern populations of the Eurasian blackcap are better interpreted as an adaptive response to altitudinal migration than as a consequence of regional food availability. [ABSTRACT FROM AUTHOR]

The ecological conditions that a bird experiences during any stage of its life cycle may have consequences that become manifested at later life stages, and these 'carry-over effects' may be major components of variance in individual performance. Condition-dependent feather growth rate, as assessed by growth bars width (GBW), provides a unique, though largely under-exploited tool to investigate carry-over effects of ecological conditions and individual physiological state during molt. In this study of breeding barn swallows Hirundo rustica, which undergo a single complete annual molt of tail and wing feathers during wintering in sub-Saharan Africa, we first show that old (≥ 2 yr) females have larger GBW than old males and yearlings. GBW was smaller with larger infestations by common ectoparasites of barn swallows, independent of the swallows' age or sex, and larger GBW was associated with a higher index of body condition during the breeding season in males. Larger GBW predicted higher seasonal reproductive output of older males, but not reproductive output of younger males or females of any age class, with this higher reproductive output of older males mediated by higher offspring fledging success. However, no relationship with GBW was observed for seasonal reproductive output of males in the spring preceding the winter when the feathers were grown. Hence, this study suggests that the analysis of the rate of feathers growth ('ptilochronology') has a larger potential to serve as a powerful tool in the study of carry-over effects than has been appreciated to date. Specifically, the present results support the idea that conditions experienced during wintering in Africa and proximately reflected by GBW have carry-over effects on body condition and breeding success. These effects are sex and age specific, being more pronounced in older males, possibly as a consequence of differences in annual time routines and susceptibility to extrinsic factors among sex and age classes. [ABSTRACT FROM AUTHOR]

The importance of plumage colour as an indicator of individual quality and the basis of sexual selection has long been recognized. Of the three generally distinguished classes of plumage colours, melanin-based ornaments are traditionally considered to provide less reliable information than carotenoid-based traits. However, the role of structural ornaments in multiple signalling systems has rarely been examined, and no study has compared the information content and role of the three ornament types simultaneously. Here we investigated three plumage ornaments in great tits Parus major: the size of the melanin-based breast stripe, the carotenoid-based colour of the yellow breast and the structurally based reflectance properties of the black crown. We worked on both the mechanistic and the functional levels. First, we assessed the dependence of ornaments on body condition during moult using ptilochronology. Second, we estimated assortative mating for these traits, as a measure of mutual sexual selection. Only the spectral attributes of crown feathers correlated with body condition during moult. However, breast stripe size was related to age, while the brightness of the yellow breast indicated body size. Relative crown ultraviolet reflectance was much higher in males than in females. Assortative mating was strongest for crown ultraviolet reflectance, but composite measures suggest that a system of multiple sexually selected traits with different information content may work in this population. These data support the accumulating evidence that the condition-dependence of melanin and carotenoid coloration is not qualitatively different. They also suggest that more research should target the reflectance properties of dark plumage areas in general, and ultraviolet crown ornamentation in tits in particular. [ABSTRACT FROM AUTHOR]