The Effect of Varying Pair Numbers on Reproduction and Use of Space in a Small Herring Gull Larus argentatus Colony

Abstract

In colonial Herring Gulls, neighbours are often the main cause of lowered reproductive output. Previous studies have claimed a relationship between nesting success and nest-density. I monitored breeding performance in a small Herring Gull colony for 7 years (1982-1988). During the study, pair numbers varied between 26 and 47. This variation produced pronounced differences in the spatial circumstances for breeding, as mean neighbour distances varied between years. In general, the colony became less dense as pair numbers decreased, but during one year the pairs clustered more tightly than a linear relationship would have predicted. I also found a strong relationship between the number of fledged chicks produced per pair and density. Production ranged between 1.22 and 2.37 fledged young pair-, being depressed in high-density years. In high-density years, more chicks disappeared from the nests, indicating that intraspecific predation was higher in these years. I was able to rule out some other factors which may have affected production. The results show that Herring Gulls sometimes breed at densities that are clearly suboptimal, and emphasize the importance of colony selection.