Down feather morphology reflects adaptation to habitat and thermal conditions across the avian phylogeny

Down feathers are the first feather types that appear in both the phylogenetic and the ontogenetic history of birds. Although it is widely acknowledged that the primary function of downy elements is insulation, little is known about the interspecific variability in the structural morphology of these feathers, and the environmental factors that have influenced their evolution. Here, we collected samples of down and afterfeathers from 156 bird species and measured key morphological characters that define the insulatory properties of the downy layer. We then tested if habitat and climatic conditions could explain the observed between-species variation in down feather structure. We show that habitat has a very strong and clearly defined effect on down feather morphology. Feather size, barbule length and nodus density all decreased from terrestrial towards aquatic birds, with riparian species exhibiting intermediate characters. Wintering climate, expressed as windchill (a combined measure of...