Population Dynamics of Common Guillemots Uria aalge on Skomer Island, Wales

The Common Guillemot population on Skomer Island, Wales, has recently increased following a long period of decline. We measured population parameters in 1985-89 and compared them with parameters measured in 1973-75 to identify the processes responsible for these population changes. Productivity increased significantly from 0.72 chicks pair-1 yr-' in 1973-75 to 0.79 chicks pair-1 yr-' in 1986-87. Adult survival was higher in 1986-89 (94.0%) than in 1973-75 (91.5%), but the difference was not significant. Information is not yet available to determine whether survival to breeding age, and therefore recruitment, has also increased. A model of population growth is presented, incorporating observed changes in productivity and adult survival, and employing three values for survival to breeding. A close fit to observed population changes was obtained with a survival rate to first breeding at five years of 41.1%, the highest survival rate previously recorded for Common Guillemots.