Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds

Abstract

Water birds have contour feathers in contact with water that show in their distal one-third adaptations to water repellency, resistance to water penetration and forceful impact with water. These qualities vary according to their intimacy with open water. In this study, the geometry of this part of the feather was examined to detect additional features that would affect viscous drag in water. The length-to-width ratio was measured and used to calculate the viscous drag coefficients for 48 water birds and, for comparison, 12 land birds. The lowest values for the drag coefficient were observed for birds with foraging niches as diving and swimming, followed by plunging, surface feeding, aerial and ground feeding. Land birds with no open water in their habitat had the highest drag coefficients. Three statistical approaches were used to validate the results. Allowing for the phylogenetic relatedness of the 60 species obscured any significant differences that may exist, but a non-parametric analysis that does not assume the conditions of equal sample size and variance turned out to be the most appropriate method for our data set.