Viability costs of male tail ornaments in a swallow

Abstract

MALE sexual ornaments that increase mating success may evolve even when they decrease other components of fitness such as survival^1–9. But natural covariation between the apparent level of investment in such ornaments and fitness components such as mating success, fecundity and survival, does not provide incontrovertible evidence that the ornaments are costly, because uncontrolled variables such as overall health⁵ may affect several components of fitness at the same time^10–12. By experimental manipulation of male tail length in the monogamous swallow, Hirundo rustica, however, the effects of tail endorment can be tested directly. I show here that in such experiments, females prefer males with elongated tails over those with shortened tails¹³, but that males with experimentally elongated tail ornaments captured smaller, less profitable prey than those with shortened tails. Impaired foraging efficiency of tail-elongated males increased the frequency of fault bars in their tail feathers, probably as a result of food deficiency during moult. Males with experimentally elongated tail ornaments also decreased their natural tail size during moult, thereby causing a fitness loss in terms of delayed breeding and a reduced annual production of offspring resulting from reduced sexual attractiveness during the following year.