The breeding biology of the Herring gull (larus argentatus)

The Herring Gull (Larus argentatus) breeding population on the Isle of May, Scotland, has been increasing at an annual rate of 13.3 per cent since 1907 when the first pair to nest were recorded. The breeding biology was studied from 1966-1969 to ascertain the relationship between nesting success and the timing of the breeding season in a species with an apparently favourable food supply. It was possible to extend the breeding season by widespread egg removal in various sub-colonies causing re-laying until mid-July. Clutch-size and egg-size in the control areas decreased through the season, and continued to do so in the delayed areas. However, hatching success and pre-fledging mortality were correlated with nesting synchronisation rather than the time of hatching, Predation, and particularly cannibalism, were the primary cause of this relationship since cannibals killed a higher proportion of early and late hatched chicks. This creates a selective advantage for laying in the middle of a well defined breeding season. There was little evidence to suggest that late breeding was a disadvantage providing that this delay applied to the group as a whole. Various aspects of chick mortality indicate a possible difference in the reproductive drive of the adults rather than an effect of food shortage. The third chick to hatch in a brood suffers a much higher post-hatching mortality than either of its siblings. An egg transfer experiment showed that both the hatching sequence and the comparative smallness of the c-egg contributed to this differential mortality, as egg size and chick mortality are closely correlated. The period between incubation and parenthood involves behavioural changes that some adults find difficulty in executing, and the mortality differences and the incidence of kronism maybe examples of this inability of adults to adjust to a complete brood.