The Feather Structure of Dippers: Water Repellency and Resistance to Water Penetration

Abstract

Dippers (Cinclidae) are the only passerines that find their prey almost exclusively under water. We examined the breast feathers of the five species of dippers for barb diameter and spacing. These results were compared with those measured for other bird families, both aquatic and terrestrial, to detect the presence of any adaptation in the structure of their plumage to their aquatic lifestyle. Barb diameters (2r) were about the same for all dipper species at ∼25 µm, but spacing (2d) varied from ∼150 to 215 µm. Dipper feathers exhibited large values for (r d)/r, the structural requirement for excellent water repellency, comparable to that of many other terrestrial birds including starlings (Sturnidae), swifts (Apodidae), and nightjars (Caprimulgidae). A slightly improved resistance to water penetration, indicated by a smaller value for (r d)/r, was observed for White-throated Dipper (Cinclus cinclus), whereas increased water repellency was found for both American (C. mexicanus) and Brown dippers (C. pallasii). Each of these three species is known to dive. No adaptations were observed for White-capped (C. leucocephalus) and Rufous-throated (C. schulzi) dippers, neither of which dive. The breast feathers of dippers have evolved excellent water repellency, but diving dippers show further adaptations for improved resistance to water penetration and water repellency to allow under-water foraging.