SEXUAL SELECTION IN THE BARN SWALLOW HIRUNDO-RUSTICA .4. AERODYNAMIC ADAPTATIONS

Secondary sexual characters are assumed to be costly to produce and ma intain, and this will select for morphological modifications that redu ce the magnitude of such costs. Here we test whether a feather ornamen t, the sexually exaggerated outermost tail feathers of male barn swall ows Hirundo rustica, a trait currently subject to a directional female mate preference, and other aspects of the morphology used for flight have been modified to increase aerodynamic performance. This was done by making comparisons among sexes within populations, among individual s varying in tail length within populations, and among populations fro m different parts of Europe. Male barn swallows experienced reduced dr ag from their elongated tail feathers by morphological modifications o f the ornamental feathers as compared to females. Morphological featur es of the outermost tail feathers were unrelated to tail length in bot h males and females within populations. Wing and tail morphology (leng th of central tail feathers and wings, wing span, wing area, wing load ing, and aspect ratio) was modified in males compared to females. Barn swallows with long tails had morphological tail and wing modification s that reduced the cost of a large ornament, and similar modifications were seen among populations. The costs of the exaggerated secondary s exual character were therefore reduced by the presence of cost-reducin g morphological modifications. The assumptions of reliable signalling theory, that signals should be costly, but more so to low than to high quality individuals, were not violated because long-tailed male barn swallows had the largest cost-reducing morphological characters.