Recruitment to a seabird population depends on environmental factors and on population size

Summary1  A novel capture–mark–recapture (CMR) method was used to build a multistate model of recruitment by young birds to a breeding population of common guillemots Uria aalge on the Isle of May, Scotland. Recruitment of a total of 2757 individually marked guillemots over 17 years was modelled as a process where individuals had to move from an unobservable state at sea, through a nonbreeding state present in the colony, to the breeding state. The probabilities of individuals returning to the colony in a given year, at age 2 and 3–4 years, were positively correlated with an environmental covariate, the winter North Atlantic Oscillation index (WNAO) in the previous years. 2  For 2 year olds, there was a negative relationship with breeding population size, suggesting that density dependence operated in this colony through limitation of food or some other resource. 3  Survival over the first 2 years of life varied with cohort, but was unrelated to the WNAO. Mean survival over this 2‐year period was high at 0·576 (95% CI: 0·444; 0·708). 4  This high survival, combined with a low ‘local’ survival after age 5 years of 0·695 (0·654; 0·733) and observations of Isle of May chicks at other colonies, suggests that most surviving chicks return to the natal colony before deciding whether to recruit there or move elsewhere.