Population cycles and kin selection in Red Grouse Lagopus lagopus scoticus

Populations of Red Grouse Lagopus lagopus scoticus often show cyclic fluctuations in numbers. There is good evidence from previous work that, each year, densities are limited by the territorial behaviour of cocks. Therefore, changes in territorial behaviour and associated territory size might cause changes in density from one year to the next. Demographic measurements and a population experiment were consistent with a model which explained cycles as a consequence of changes in behaviour resulting from kin selection: differential behaviour between kin and non‐kin. The demographic measurements presented here covered 16 years and two cyclic‐type fluctuations, and the experiment lasted 10 years. Long‐term studies are essential if lengthy processes are to be understood reliably.