AN EXPERIMENTAL-STUDY OF THE EFFECTS OF TIMING OF BREEDING ON THE REPRODUCTIVE SUCCESS OF COMMON GUILLEMOTS (URIA-AALGE)

(1) The reproductive success of 474--574 pairs of common guillemots was studied from 1985 to 1987 on Skomer Island, Wales. (2) Laying was highly synchronized, and within years productivity decreased seasonally from shortly after the period of peak laying. (3) The timing of breeding of one group of forty-five pairs was experimentally delayed by 13 days to test whether timing of breeding relative to calendar date or relative to other pairs was the factor causing the seasonal decline in productivity. The results showed that timing relative to other individuals rather than calendar date was important. (4) Naturally late-breeders provisioned chicks at a lower rate than early breeders, but there was no evidence for a seasonal deterioration in food availability. (5) The effects of physical characteristics, breeding density and synchrony on productivity were examined in a logistic regression analysis for 567 breeding sites. Only density and synchrony had significant effects. Productivity increased with density and decreased for late breeders. (6) Predation was the main cause of breeding failure. High-density breeding reduced vulnerability to predators. Synchrony was important for the benefits of high-density breeding to apply throughout the breeding period, and also for swamping predators.