Within- and between-year variation in the
juvenile survival of Common Guillemots Uria aalge
Harris MP, Frederiksen M & Wanless S (2007) Ibis 149:472-481
We studied juvenile survival of 20 cohorts of Common Guillemots Uria aalge
chicks colour-ringed on the Isle of May, Scotland, using both live observations
at the colony and dead recoveries, allowing an estimation of fidelity of the colony
as well as survival. In this seabird, chicks leave the colony when only partly grown
and are cared for by the male parent for several week afterwards. First-year
survival varied strongly between cohorts, with a mean of 56% (range 30% - 91%).
We did not identify any covariates which could explain this variation, whether
relating to climate, population size or prey density. Survival was low during two
regime shift episodes in the North Sea (1987-1990 and 2000 onwards). Early
hatched chicks were substantially more likely to survive than those hatching
later in most years, whereas body condition at ringing had no detectable effect.
Ringing recoveries indicated that mortality was highest in mid-winter, i.e. well
after the cessation of paternal care. These results do not support the hypothesis
that variation in prey quantity or energy content before fledging is a primary
driver of variation in juvenile survival. Rather, it seems that chicks of high-quality
parents are more likely to survive, as high-quality females tend to lay earlier in
the season, and high-quality males presumably are better able to prepare their
chicks to survive their first winter at sea. Very few (4%) Guillemots emigrated
permanently before age 3, but from age 5 onwards 25-30% of the birds annually
left the colony or otherwise became unobservable.
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