SEXUAL ORNAMENTATION AND IMMUNOCOMPETENCE IN THE BARN SWALLOW

The handicap hypothesis of honest signaling suggests that secondary se xual characters reliably reflect phenotypic or genotypic quality of si gnalers. This hypothesis is based on the assumptions that signals are costly to produce and/or maintain and the cost of a given level of sig naling is higher for low quality than for high quality signalers. We t ested these assumptions in a field experiment in which the size of a s econdary sexual character [tail length in male barn swallows (Hirundo rustica)] was experimentally manipulated. Males were randomly assigned to tail elongation, tail shortening, or two control treatments (tail manipulation, or just capture, ringing, and handling). Male barn swall ows were challenged with an injection of sheep red blood cells, and bl ood was sampled on the day of first capture and after 3 to 4 weeks for determination of concentrations of gammaglobulins. Tail-elongated mal es did not increase levels of gamma-globulins while males of the other three groups demonstrated increases. Analyses of variation in gamma-g lobulins within treatment groups revealed a positive correlation betwe en gammaglobulins and original tail length among males with elongated tails. These results suggest that tail length imposes an immunocompete nce cost on males, and that males with naturally long tails are differ entially better able to cope with this cost.