EFFECTS OF TESTOSTERONE ON BREEDING DENSITY, BREEDING SUCCESS AND SURVIVAL OF RED GROUSE

Territorial intolerance can limit bird numbers by preventing subordina tes from holding territories and breeding. We report the first control led and replicated experiment in a natural population of territorial a nimals which shows that population density can be decreased by implant s of testosterone inducing increased aggressiveness. Implanted territo rial cocks expanded their territories at the expense of unimplanted ne ighbours, some of which left the study area. The probability of an uni mplanted cock leaving the study area increased with his number of impl anted neighbours. Implanted cocks had better breeding success but poor er subsequent survival than control cocks.