A comparative study of aerodynamic function and flexural stiffness of outer tail feathers in birds

To transmit aerodynamic forces to the body, tail feathers should be stiff to resist lift forces with minimum deformation. Because aerodynamic theory predicts that such feathers do not produce lift forces beyond the point of the maximum continuum width of the tail, species with deeply forked tails should not require stiff outer rectrices distal to that point. I tested this prediction by comparing the relative thickness of the outer rectrix rachis between species with deeply forked tails to those with triangular or shallowly forked tails. Eleven pairs of closely related species belonging to families Fregatidae, Phalacrocoracidae, Accipitridae, Sternidae, Caprimulgidae, Trochilidae, Coraciidae, Tyrannidae, Cotingidae, and Hirundinidae were compared. All but one of the phylogenetically independent comparisons showed that the species with triangular or shallowly forked tails have higher relative rachis thickness than their deeply forked relatives. In addition, nine out of eleven of the species with deeply forked tails showed a proportionately greater increase in relative rachis thickness from distal to proximal parts of the feather. In contrast, triangular and shallowly forked tails showed an approximately linear relation between relative rachis thickness and relative rachis length. These results considered together are consistent with the idea that the distal part of outer rectrix rachis in species with deeply forked tails has not been selected to resist lift forces and may be adaptively reduced to attenuate the costs of a hypertrophied ornament.