Effects of Testosterone on Breeding Density, Breeding Success and Survival of Red Grouse

Territorial intolerance can limit bird numbers by preventing subordinates from holding territories and breeding. We report the first controlled and replicated experiment in a natural population of territorial animals which shows that population density can be decreased by implants of testosterone inducing increased aggressiveness. Implanted territorial cocks expanded their territories at the expense of unimplanted neighbours, some of which left the study area. The probability of an unimplanted cock leaving the study area increased with his number of implanted neighbours. Implanted cocks had better breeding success but poorer subsequent survival than control cocks.