Professor John C. Coulson - Gulls

Regional survival rates

The survival rates of Black-headed Gulls ringed as pre-fledged chicks or as adults in different regions of Britain ( Table 19) show little regional variation, and the small differences are all within the tolerances of the methods used. In general, the mortality rate in the first year of life is about double that of the birds once they become adults. However, the estimates for Ireland indicate that Black-headed Gulls here had both the lowest first-year and adult survival rates, and the lowest adult life expectancy. These figures are consistent with the appreciable current decline in adult numbers nesting there. The highest adult survival rate and adult life expectancies were in Scotland, which is consistent with the increase in numbers reported nesting there between 1985 and 2000.

TABLE 19. Estimates of the annual survival rates of adult and first-year Black-headed Gulls, and life expectancy of adults, in Britain and Ireland according to the area in which they hatched. Data are restricted to those birds ringed between 1950 and 1980.

MOULTING

There is surprisingly little information on the progress of the primary wing moult of Black-headed Gulls. The start of moult commences at the innermost primary, with the old feather usually dropped by adults in June (the earliest recorded date is 26 May), at a time many have already hatched chicks. Since the inner primaries are relatively short, the start of moult at this stage will have little effect on flight and the ability of parents to search for food for the young. After incubation is complete, the primary moult progressively spreads along the wing, until the last (outer) primaries are dropped in September and replaced by fully grown feathers by mid-October. In all, the primary moult takes three to three-and-a-half months from the first feather being shed to the longest primaries becoming fully grown.

The moult of the primary feathers by immature one-year-olds starts about a month earlier than in adults and is completed by September – a three-month period, which is generally shorter than that taken by the adults. The period when the outer and longer ninth and 10th primaries feathers are missing (August for first-year birds and September for adults) probably interferes most with efficient flight, and so this may inhibit long-distance migration until the new outer primaries have grown.

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