Haematocrit correlates with tail ornament size in three populations of the Barn Swallow (Hirundo rustica)

Advertisement models of sexual selection propose that exaggerated male secondary sexual characters could reliably signal male health and vigour by having the function of ‘handicaps’, i.e. characters that impose a cost to their bearer (Zahavi 1975, 1977; Heywood 1989; Grafen 1990). A commonly cited example of such characters is the ornamental tails of several bird species (Darwin 1871; Fisher 1930; Pomiankowski 1988). Extravagant tails apparently confer an advantage in the mating process because, by having a long tail, males are preferred by prospecting females. On the other hand, tail ornaments may be disadvantageous in contexts other than mate choice. It has been proposed, for example, that exaggerated tails may be costly because high androgen profiles, necessary for the development of male secondary sexual characters, have a suppressive effect on the immune system, thus reducing host immune defences against parasites, or exert a direct effect on parasites (Beckage 1983; Grossman 1985; Lawrence 1991; Folstad & Karter 1992). Large ornaments may be more conspicuous to predators than small ones, thus increasing predation risk. Moreover, aerodynamic costs may also increase with ornament size (e.g. Thomas 1993). This will be reflected in higher energy expenditure for flying or in lowered foraging efficiency determined by reduced Functional Ecology 1997 11, 604–610