Mechanical and structural adaptations to migration in the flight feathers of a Palaearctic passerine

Current avian migration patterns in temperate regions have been developed during the glacial retreat and subsequent colonization of the ice-free areas during the Holocene. This process resulted in a geographic gradient of greater seasonality as latitude increased that favoured migration-related morphological and physiological (co)adaptations. Most evidence of avian morphological adaptations to migration comes from the analysis of variation in the length and shape of the wings, but the existence of intra-feather structural adjustments has been greatly overlooked despite their potential to be under natural selection. To shed some light on this question, we used data from European robins Erithacus rubecula overwintering in Campo de Gibraltar (Southern Iberia), where sedentary robins coexist during winter with conspecifics showing a broad range of breeding origins and, hence, migration distances. We explicitly explored how wing length and shape, as well as several functional (bending stiffness), developmental (feather growth rate) and structural (size and complexity of feather components) characteristics of flight feathers, varied in relation to migration distance, which was estimated from the hydrogen stable isotope ratios of the summer-produced tail feathers. Our results revealed that migration distance not only favoured longer and more concave wings, but also promoted primaries with a thicker dorsoventral rachis and shorter barb lengths, which, in turn, conferred more bending stiffness to these feathers. We suggest that these intra-feather structural adjustments could be an additional, largely unnoticed, adaptation within the avian migratory syndrome that might have the potential to evolve relatively quickly to facilitate the occupation of seasonal environments.

Higher Nest Predation Favors Rapid Fledging at the Cost of Plumage Quality in Nestling Birds

High predation risk can favor rapid offspring development at the expense of offspring quality. Impacts of rapid development on phenotypic quality should be most readily expressed in traits that minimize fitness costs. We hypothesize that ephemeral traits that are replaced or repaired after a short period of life might express trade-offs in quality as a result of rapid development more strongly than traits used throughout life. We explored this idea for plumage quality in nestling body feathers, an ephemeral trait. We found a strong trade-off whereby nestlings that spend less time in the nest produced lower-quality plumage with less dense barbs relative to adults across 123 temperate and tropical species. For a subset of these species ( n=67 ), we found that variation in the risk of nest predation explained additional variation in plumage quality beyond development time. Ultimately, the fitness costs of a poor-quality ephemeral trait, such as nestling body feathers, may be outweighed by the fitness benefits of shorter development times that reduce predation risk. At the same time, reduced resource allocation to traits with small fitness costs, such as ephemeral traits, may ameliorate resource constraints from rapid development on traits with larger fitness impacts.

Silver spoon effects on plumage quality in a passerine bird

A silver spoon effect means that individuals who develop under favourable circumstances enjoy a fitness or performance advantage later in life. While there is large empirical support for silver spoon effects acting on different life-history traits in birds, such as survival and reproduction, the evidence for the carry-over effects of rearing conditions on the quality of future plumage generations is lacking. Here, we examined whether abilities of individuals to undergo extensive post-juvenile moult may depend on the quality of juvenile plumage developed during the nestling phase in a small passerine showing large individual variation in the extent of post-juvenile moult, the greenfinch (Carduelis chloris). We found that high structural quality and carotenoid chroma of juvenile feathers were positively linked to the extent of post-juvenile moult in this species, thus allowing young birds to attain more adult-like plumage. Silver spoon effects mediated by the juvenile plumage quality were also found to have other fitness-related consequences, as individuals with high-quality juvenile feathers were in better condition during their first winter. As far as we are aware, the results provide the first correlative evidence for a silver spoon effect acting on general plumage quality in birds.