The Importance of Relative Laying Date on Breeding Success of the Guillemot Uria aalge

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Abstract
We studied the breeding of Guillemots in five areas on the Isle of May for five years. Chicks which hatched early had significantly longer fledging periods than late chicks. This trend was related to the timing of breeding in the colony in general and not to the relative timing of areas. Within each year there was a significant decline in breeding success due mainly to a decrease in hatching success but partly to a decline in chick success later in the season. The main factor influencing the chances of successful breeding was the date the egg was laid relative to the laying dates of pairs in the surrounding area. Pairs in areas where breeding was late did, on average, as well as those in early areas. Neither predation nor deterioration of feeding conditions was considered a likely cause of the decline in hatching success although the actual reason remained undertermined. The changes of a pair replacing a lost egg was also related to the relative laying date. The time a lost egg had been incubated had no effect on the chances of it being replaced. To maximize its lifetime reproductive success a Guillemot should select the area in which it can breed earliest.