Correlating feather structure, wettability, and robustness with ecological behavior of aquatic birds

Abstract

In nature, aquatic birds can interact with water without their feathers being easily wetted; some species dive tens of meters and emerge to spread their wings to dry. In past studies attempting to connect such ecological behavior and feather structure, the typical approach of microscopy has demonstrated the difficulty in characterizing specimens as delicate and complex as feathers by visual techniques alone. In this work, the question was addressed of how various species balance the wettability problem with the need to dive to various depths or to remain on or near the water surface as dictated by their feeding habits. Texture of wing feathers from six different species of aquatic birds was characterized by measuring contact angles and applying the previously developed framework of the effective spacing ratio, D*, and robustness factor, A*, according to the Cassie-Baxter relation for composite interfaces. This "effective microscopy" technique was successfully employed to assess the wettability and robustness of bird feather textures. The observable water-related behaviors of diving, wing-spreading, shallow foraging, and dabbling for the species studied were explained as partly determined by feather structure, exhibiting effective- D* analysis as an adequate technique for characterizing complex, textured surfaces, fabricated or natural.