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 the ambient temperature and wind speed) had limited
effects on down morphology, colder climate only being associated with
higher nodus density in dorsal down feathers. Overall, an aquatic
lifestyle selects for a denser plumulaceous layer, while the effect of
harsh wintering conditions on downy structures appear limited. These
results provide key evidence of adaptations to habitat at the level of the
downy layer, both on the scale of macro- and micro-elements of the
plumage. Moreover, they reveal characters of convergent evolution in the
avian plumage and mammalian fur, that match the varying needs of
insulation in terrestrial and aquatic modes of life.