HEMATOCRIT CORRELATES WITH TAIL ORNAMENT SIZE IN 3 POPULATIONS OF THEBARN SWALLOW (HIRUNDO-RUSTICA)

1. Handicap models of sexual selection propose that male ornaments are indicators of male quality and that honesty is enforced by the costs imposed by the exaggerated ornamental traits. In long-distance migrato ry birds that feed on the wing, the aerodynamic cost of exaggerated or namental characters should be particularly high because the size of th e ornaments deviates from the natural selection optimum. During migrat ion, birds are expected to raise their oxygen consumption in relation to the energetic demands imposed by their morphology. An increase of h aematocrit is an adaptive response to enhance oxygen uptake and effici ency of transfer to the muscular tissues during spells of intense musc ular activity. 2. The change of haematocrit of Barn Swallows (Hirundo rustical after their arrival to the breeding sites, and the relationsh ips between haematocrit values recorded after migration and the size o f ordinary and sexually selected morphological characters in three Bar n Swallow populations were analysed. 3. Males had higher haematocrit v alues than females. Individual haematocrit values declined after arriv al to the breeding sites. Haematocrit values of males were significant ly and positively correlated with the size of their ornamental tail bu t not correlated with other characters, thus suggesting that well-orna mented males, in order to arrive early, have to raise their haematocri t above the level of short-tailed males. 4. Males and females of simil ar tail length did not differ in their haematocrit, thus suggesting th at sexual dimorphism in haematocrit might be functionally related to d imorphism in tail length. 5. Our results are consistent with the handi cap principle because long-tailed males experience lower mortality and larger seasonal reproductive success compared with short-tailed males .