THE ASYMMETRICAL COST OF TAIL ELONGATION IN RED-BILLED STREAMERTAILS

Ornaments used in signalling should vary in size with signaller qualit y. It has also been suggested that high-quality individuals should be less susceptible to asymmetrical development than low-quality individu als. It has been claimed that the degree of asymmetry in ornaments cou ld be used by receivers to assess signaller quality. If these claims a re correct, high-quality individuals should produce large symmetrical ornaments, and low-quality individuals small, asymmetrical ones. Negat ive relations between tail length and asymmetry have been demonstrated and taken as support for these predictions. However, these results co uld occur if individuals growing long tails were constrained to more s ymmetrical development. This mechanical constraints hypothesis predict s that the costs of a given amount of asymmetry should increase with i ncreasing tail length and, when tails are asymmetrical, the costs of i ncreasing tail length should rise faster than when symmetrical. To tes t these predictions, we manipulated tail streamer length and symmetry of male red-billed streamertails (Trochilus polytmus) which were relea sed through a maze forcing the birds to manoeuvre to avoid collision. We show that if streamer length is experimentally reduced then manipul ations of symmetry have no effect on the ability of the birds to negot iate this maze. However, when streamers are experimentally elongated, the time taken to negotiate the maze and the number of collisions incr ease with the degree of the asymmetry manipulation. This demonstrates that individuals producing long tails may be forced into greater symme try to minimize costs.