Changes in the Breeding Biology of the Herring Gull (Larus argentatus) Induced by Reduction in the Size and Density of the Colony

Publisher Website
Google Scholar
Abstract
Between 1972 and 1981, the Nature Conservancy Council culled large numbers of breeding herring gulls L. argentatus on the Isle of May, Scotland [UK]. As a result of the culling, the population was reduced to about a quarter of the 1972 level. By 1981, the cull had only marginally reduced the total area occupied by breeding gulls but the density of breeding birds had been markedly reduced. The age of recruitment has decreased since the start of the cull. Prior to the cull, there were no records of 3rd yr birds breeding or even holding territory, but now a few 3rd yr birds breed each year. The main change in the recruitment to the breeding population was in the increased proportion of birds breeding at 4 yr of age. The mean age of recruitment has decreased by over a year. Recruitment to the colony has remained high in most years but there is not a close relationship between the proportion of the breeding population culled in 1 yr and the change in the breeding population in the next year, suggesting that the amount of reecruitment varies from year to year. This variation is also caused by intermittent breeding in some years or breeding birds moving into other colonies. There is evidence that an increased proportion of the young reared on the Isle of May are returning there to breed. This is not due to an increased survival in young herring gulls (the survival rate is already very high) but to a smaller proportion emigrating and recruiting into other colonies. Both body weight and wing length of breeding herring gulls have increased as the population size decreased. This increase has taken place within individual cohorts and it is suggested that individual birds have gained in size. These effects result from the reduction of intraspecific competition either on the breeding or in the feeding areas. Younger herring gulls lay smaller eggs. The effect of culling has been to markedly reduce the average age of the breeding birds but, despite this, the mean egg size has increased over that recorded there in 1967 and 1972, before the cull commenced. This also reflects an improvement in the condition of the breeding birds. Several factors associated with the population dynamics of the herring gull have changed as a result of reducing the density (and number) of breeding birds. These changes can be regarded as density-dependent effects, all of which tend to compensate for the much increased mortality rate of adult herring gulls induced by culling.