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Résumé :
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This paper presents the results of a 29-year study (1983-2011) of the biology and population dynamics of Razorbills Alca torda breeding on Græsholmen in the central Baltic Sea. During the study years, the breed- ing population increased from c. 250 pairs in 1983 to 1200 pairs in 2011 (380%). Though variable, the mean annual survival was high (93-95%) for breeding adults, as was the overall survival of immature birds (57.7% up to the age of four years) returning to breed on Græsholmen. Mean age of first breeding was 4 years, but around 23% of the birds started to breed when 3 years old. First and second year birds visited the colony regularly but did not breed. Generally, older females laid larger eggs earlier in the breeding season and had a higher breeding success than females breeding for the first time. Overall, breeding success averaged 71-75% in seven study plots. Mean hatching date was 19 June (23 May – 6 August). Fledging chicks were recorded in the period 22 June – 15 August and the mean fledging age was 19 days. Chicks fledging early in the season were significantly larger and heavier than late fledging chicks, but no difference was found in post-departure survival. Sprats Sprattus sprattus were the main food (90%) supplied to the chicks. Average rate of non-breeding among adult birds with previous breeding experience was 3%. Average divorce rate was 14.5% and was highest (26%) among the youngest and lowest (3.1%) among the oldest breeding birds. Around 50% of the ringed chicks returning to breed on Græsholmen did so in their natal subcolony. Once settled, local nest-changes over the years occurred among 44% of the birds, but
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