Sexual ornamentation and immunocompetence in the barn swallow

The handicap hypothesis of honest signaling suggests that secondary sexual characters reliably reflect phenotypic or genotypic quality of signalers. This hypothesis is based on the assumptions that signals are costly to produce and/or maintain and the cost of a given level of signaling is higher for low quality than for high quality signalers. We tested these assumptions in a field experiment in which the size of a secondary 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 swallows were challenged with an injection of sheep red blood cells, and blood was sampled on the day of first capture and after 3 to 4 weeks for determination of concentrations of gammaglobulins. Tail-elongated males did not increase levels of gamma-globulins while males of the other three groups demonstrated increases. Analyses of variation in gamma-globulins within treatment groups revealed a positive correlation between gammaglobulins and original tail length among males with elongated tails. These results suggest that tail length imposes an immunocompetence cost on males, and that males with naturally long tails are differentially better able to cope with this cost.