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- Title
Diagnosing a decline in return rate of 1-year-old cormorants: mortality, emigration or delayed return?
- Authors
Frederiksen, Morten; Bregnballe, Thomas
- Abstract
1. In long-lived birds with delayed recruitment, variation in prebreeding population parameters is difficult to study, although potentially important. Cormorants (Phalacrocorax carbo sinensis) often return to their natal colony when 1 year old, enabling direct study of first-year survival and emigration. Density-dependent regulation mechanisms may be strong at this stage of the life cycle. 2. In the Danish cormorant colony Vorsø, 11 000 chicks have been colour-ringed between 1977 and 1997. Returning birds have been resighted in the colony and dead recoveries have been collected. Since 1990, the proportion of a cohort observed in the colony as 1-year-olds (return rate) has declined from 0·40 to 0·10. Possible explanations include increased mortality, increased emigration and a later age of first return to the colony. Breeding success has also declined strongly as a consequence of low food availability. 3. We used capture—recapture analysis of recovery and resighting data to investigate variation in first-year survival, emigration and resighting probabilities. Survival fluctuated widely (0·42-0·75, mean 0·58); emigration increased in the 1990s from 0·05 to 0·15; resighting probability declined from 0·75 to 0·20 after 1990. 4. First-year survival was particularly low in 1993 and 1996. The causes of the year-to-year variation were not clear. Survival may have been affected by food availability during the post-fledging period. 5. The declining return rate was caused mainly by decreasing resighting probability of 1-year-olds, although increasing emigration also contributed. The biological mechanism was that increasing numbers of cormorants did not return to the natal colony as 1-year-olds. We suggest that this was a consequence of low physiological condition among newly fledged young, caused by low food availability in the 1990s. 6. We conclude that declining food availability has had se...
- Subjects
DENMARK; GREAT cormorant; BIRD migration
- Publication
Journal of Animal Ecology, 2000, Vol 69, Issue 5, p753
- ISSN
0021-8790
- Publication type
Article
- DOI
10.1046/j.1365-2656.2000.00436.x