THE BIOLOGY OF OYSTERCATCHERS HAEMATOPUS OSTRALEGUS ON SKOKHOLM ISLAND, S. WALES

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Summary

A population of about 50 pairs of Oystercatchers Haematopus ostralegus nesting on Skokholm Island, S. Wales, was studied between 1963 and 1965. Other information on Skokholm Oyster-catchers over the last 25 years was also analysed.

The adults were trapped, colour-banded and sexed by bill measurements. With few exceptions, birds kept the same mates and territories from one season to the next. Apparently the male is responsible for retaining the territory. Some Oystercatchers breed at three years but most do not do so until four or five. It seems that there is a high population pressure, preventing young birds from establishing themselves. The annual adult survival in the two years was 88% and 90% respectively.

The average clutch-size on Skokholm is known for many years, and varied annually between 2.5 and 3.3. The number of c/4 varied greatly from year to year. There was a seasonal decline in mean clutch-size and also in egg and nest success. Predation, especially by Lesser Black-backed Gulls, was the main cause of egg loss.

In 1963, 36.7% of hatched chicks fledged and 59% in 1964. The larger clutches were more successful and produced more surviving young than did the more numerous smaller clutches. Early hatched young are more successful than later young.

The average yearly mortality of birds between fledging and breeding was 40%.

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