Sources of distinctness of juvenile plumage in Western Palearctic passerines

Juveniles of many avian species possess a spotted or mottled body plumage that is visually distinct from the plumage of adults. In other species, however, juveniles fledge with a body plumage that is just a pale representation of adult female plumage. The reasons for this variation are poorly understood. Several hypotheses concerning social (parent–offspring, adult–juvenile, juvenile–juvenile), ecological (predation risk) and physiological (costs of plumage development) implications of juvenile body plumage are presented in relation to predictions concerning associations with certain ecological and life-history attributes of avian species. In the present study, we conduct a phylogenetically corrected comparative analysis of Western Palearctic passerines looking for sources of variation in the incidence of distinct and adult-like juvenile body plumages. We scored plumages based on plates in the Handbook of the Birds of the Western Palearctic (Cramp & Perrins, 1988–1994; Oxford University Press) (HBWP) and entered body mass, migratory habits, habitat, nestling diet, breeding dispersion, gregariousness, duration of the nestling period, type of nest, conspicuousness of female plumage, and sexual dimorphism as explanatory variables, as presented in HBWP, in phylogenetic generalized least square regression analyses. One-third of the species presented distinct juvenile body plumages, which lasted on average for the first 2 months of life. Body mass, conspicuousness of female plumage, migratory habits, and habitat were significantly associated with interspecific variation in distinctness of juvenile plumage, with smaller species, more conspicuous species, migrants, and species from forested habitats showing distinct juvenile plumages with higher frequency. The phylogenetic signal was moderately high. Assuming that conspicuous adult plumage is costlier to produce than distinct juvenile body plumage (pigments, conspicuousness), the need to acquire social status among juveniles before the winter may explain the more adult-like plumage in resident species because juveniles will probably compete with individuals that they may have known during their first months of life. On the other hand, migrant juveniles may compete with a different set of individuals in winter quarters and can use savings in resources necessary for developing adult-like plumages to improve migration capacity by allocating resources to other functions. The association with habitat could be related to juveniles in open habitats participating in more extended interactions with other juveniles than in forested habitats where lower visibility may reduce the capacity to detect or respond to signals from juvenile conspecifics. More studies on this possibly crucial life stage are needed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102, 440–454.