Changes in the down colour of chicks during a population decline in red grouse.

Difficulties in using genetic markers to demonstrate that changes in gene frequency cause fluctuations in the density of animal populations are illustrated. The frequency of chicks with light-coloured down increased during 4 years when population density of red grouse declined and the inherent level of aggressive behaviour of adults increased. Variations in aggressive behaviour and down colour of individuals within years, however, were not related.