Functional evolution of facial bristle feathers in Old World passerines

Abstract Many bird species have vaneless hair-like feathers in the frontal area of the head (facial bristles). Several alternative hypotheses on the functional significance of avian facial bristles have been proposed, including aid in insect capture, eye protection, and mechanosensory sensitivity. Here, we used a phylogenetically-informed comparative approach to examine functional evolution of facial bristle feathers in Old World passerines (Passeriformes). For this purpose, we quantified the number and length of facial bristles located at the base of the upper beak (upper rictal bristles) in over 1300 birds representing 73 species and 21 families, and tested for evolutionary associations of these traits with species ecology, biogeography, life history, and morphology. We found that more numerous and longer rictal bristles have evolved in association with invertivory and omnivory. The number or length of rictal bristles was also linked to aerial foraging, foraging in low vegetation, occupancy of shrubland habitats, and migration. All these evolutionary correlations suggest that avian rictal bristles may serve multiple non-exclusive functions related to insect capture and tactile sensing. Comparisons with previous research provide support for scale dependence in avian rictal bristle evolution and decoupling of evolutionary processes at different levels of phylogenetic organization.