TIMING AND SPACING OF BROODS IN THE BLACK-HEADED GULL LARUS RIDIBUNDUS

The nests of the Black-headed Gull Larus ridibundus are closely aggregated into dense colonies and their use synchronized, these two phenomena together tending to produce a maximal clumping effect. Within such a colony however, nests were found to be spaced out to produce a non-random uniform distribution. The commonest distance between neighbouring nests was found to be about one metre, in contrast to related species. This study was concerned with two aspects of this distribution pattern; its survival value and its behavioural causation.
It was found that pairs nesting just outside the colony had a much lower breeding success than those nesting in the colony and that nests on the colony fringe had a slightly lower success than those in the centre. Pairs laying during the peak laying period had a higher breeding success than pairs laying either earlier or later in the season. Since by far the most important mortality agent was predation, it seems likely that both clustering and synchronization of nesting function as antipredator systems and arguments in favour of this are discussed.
Variations in nest-spacing within the colony were not correlated with variations in breeding success.
In the causation of the spacing between nests, territorial aggression was demonstrated to be an effective dispersion mechanism and the way in which this mechanism works was investigated in detail.
This spacing mechanism was not sufficient by itself to explain the observed densities, which were higher than one would expect from the aggression alone; there was also some tendency for birds establishing a new nest-site to cluster close to others. The interaction between this, the territorial aggression of the residents and the subsequent avoidance responses of the settling birds, can explain the nest spacing pattern and probably also the observed densities.