Aspects of the biology of the herring gull (larus atgentatus pont.)

Abstract

A study of the herring gull Larus argentatus emphasising interrelationships of population ecology, social behaviour and breeding biology was undertaken on the Isle of May, Scotland, with some comparative work in a moorland gull colony on Mallowdale Fell in the Pennines. A cull of the herring gull population, which had hitherto been increasing at 13% per annum, has been practised by the Nature Conservancy Council on the Isle of May yearly since 1972, and special attention was given in this study to the biological effects of culling. The population trends were followed in detail up to 1977, and it was shown that the annual, recruitment rate has been very variable since 1972 and there has been a shortfall in the number of young gulls predicted to join the breeding population. These have presumably moved and some ringed gulls were located breeding in other colonies. The population has been held at about 20-25% of its 1972 level since 1975. With many of the older gulls having been culled, the average age of the population had been reduced, so that by 1977 about 50% of the population was breeding for the first time. Despite a strong correlation between parental age and percentage breeding success, the average number of chicks fledged per pair in control areas was 0.82, which was as high as that recorded in a previous study by Parsons (1971, unpublished Ph.D. thesis, Durham University) on the Isle of May in the late 1960s. In addition, it was found that the median date of laying was the same as that estimated by Parsons, and that egg size (positively correlated with chick survival) had increased significantly. Experiments on recruitment by manipulating breeding density showed that at the highest densities recorded the annual recruitment rate was close to the average annual mortality rate. In areas where density had been greatly reduced, the recruitment rate was insufficient to replace annual adult mortality, and in some areas no recruitment was recorded. There was a broad, optimal breeding density of between 2 and 10 nests/l00m(^2) where highest recruitment rates took place. Much of the Isle of May was found to be at this density as a result of culling. Birds which spread their nests most uniformly were the most successful breeders, and the majority of nests were thus spaced. Aggression was found to increase with density. The rationale of gull culling has been discussed for the Isle of May, together with recommendations for future culls on the island and elsewhere.