Behaviour, ecology and recruitment of immature guillemots 'Uria aalge'

Abstract

Guillemots have been studied on the Isle of May since 1980. Between 184 and 581 individually identifiable birds were ringed as chicks each year since 1983. Cumulative known survival rates varied from 14-39% for cohorts at least 3 years of age. Survival rate was highly significantly negatively correlated with the number of hours of gale force winds in the three months after a cohort first goes to sea. Immatures did not return to the colony until at least 2 years of age. The proportion of a cohort attending the colony increased, cohorts arrived at the colony earlier in the season, and individuals were seen more often up to the age of 4-5. Experienced individuals arrived earlier, were seen more often, and were more likely to recruit than inexperienced birds of the same age. Two and 3 years olds but very few older birds visited sea rocks below the colony; all ages were seen on top ledges and on sites within the breeding colony. Immatures tended to visit the same subcolony repeatedly; older cohorts were more sedentary. Guillemots were highly philopatric to their natal subcolonies as prebreeders and recruits. Recruitment age varied from 3 years to 8+, median 6. Recruits fledged 0.26 chicks/pair compared to 0.79/pair in the breeding population as a whole. Recruits arrived earlier in the year of recruitment, and in the previous year, than same-aged birds which did not recruit. Substantial numbers of immatures from other colonies visited the Isle of May and Isle of May bred birds were observed elsewhere. Non-native immatures were seen fewer times than natives. Activity at the colony varied little between immature cohorts, but suggested increasing competitive ability with age. The results are discussed in relation to wider issues in seabird behaviour and ecology.