Comparative evidence for costs of secondary sexual characters: adaptive vane emargination of ornamented feathers in birds

We used a comparative approach, by comparing bird species with tail ornamen tation with sister taxa without ornamentation, to deduce the aerodynamic fu nction of extravagant feather ornaments and the costs Of such ornaments in birds. First, the aerodynamic function of tail feather ornaments in birds c an be deduced from asymmetry in the width of tail feather vanes, since nigh tless birds have symmetrical vanes while flying birds without feather exagg eration by sexual selection have asymmetrical vanes, Distal inner vanes at the tip of tail feathers were more narrow in ornamented as compared to nono rnamented birds, and vane asymmetry at the tip of the feather was therefore reduced in ornamented species, suggesting marginal aerodynamic function of the distal part of extravagant feather ornaments. Second, the cost of feat her ornaments due to parasite drag is proportional to the area of feathers extending beyond the maximum continuous width of the tail, and aerodynamic costs of long tails could therefore be diminished by a reduction in feather width. Consistent with this prediction, the outermost tip of feather ornam ents was narrower than the homologous character in nonornamented sister tax a, while the base of the feather had similar width in the two groups of bir ds. These results suggest that the costs of extravagant ornamentation have been diminished by a reduction in feather width, leading to a reduction in drag. Costs of feather ornaments, as demonstrated by their fine morphology, thus appear to have been extensive during the evolution of these character s.