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Post-fledging survival and sexual maturation in Common guillemots Uria aalge
- The influence of biological and ecological factors

Kadin, M. (2007) Examensarbete 2007:19, Department of Systems Ecology, Stockholm University
1. Common guillemots Uria aalge, ringed as chicks in 2000-2005, were studied when returning to their natal colony on Stora Karlsö (Baltic Sea) in 2002-2006. 390 ringed birds were observed visiting breeding ledges where their behaviour were studied in detail and used to define “early maturity”. The effects of sex and condition as chick (using fledging weight as a proxy for condition) on maturity were tested. In addition, club attendance (i.e. attendance of immature individuals on rocks adjacent to the breeding colony) was studied in 2004-2006. Observations at breeding ledges and clubs, together with ring recovery data, were used to calculate minimum post-fledging survival.

2. There was a clearly distinctive pattern in how immature birds successively developed adult behaviour (finding a suitable breeding location, establishing a pair-bond and breeding). In contrast to findings elsewhere, 1-year-olds were observed at the colony, commonly on clubs and occasionally on breeding ledges. 2-year-olds were the most frequently observed age class at breeding ledges, possibly due to a high return rate and a tendency towards substantial mobility between ledges. Some 2-year-olds repeatedly visited one specific breeding ledge, but this behaviour was more common for older birds. Establishments of pair-bonds were first seen among 3-year-olds and the first observed breeding attempts occurred at age four. No differences were found between males and females in developing adult behaviour and thus speed of maturation.

3. The more recently born cohorts (from 2003 and 2004) appeared to survive better than cohorts 2000-2002. A cold winter experienced by the earlier (2000-2002) cohorts may explain this, but it is also possible that bycatches in fisheries has been reduced in recent years.

4. There was no evidence of a correlation between chick fledging weight and post-fledging survival.

5. Contrary to expectations, guillemots defined as “early maturity”-individuals were lighter at fledging than birds defined as “normal maturity”-individuals. There are two possible explanations for these counter-intuitive results. Low quality individuals have reduced life expectancy or reproductive performance and for them, early investments in reproduction (analogous to the “early maturity” individuals) are more likely to pay off rather than saving resources for the future. Assuming that fledging weight is positively correlated to quality, individuals with low fledging weight are thus expected to mature at younger ages. An alternative explanation is that fledging weight does not reflect individual quality, but that both weight and maturity are physiologically controlled by the same mechanism, for example an endocrine factor.

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