The formation and growth of seabird colonies: Audouin’s gull as a case study

Abstract

Summary Simulations of a stochastic, individual‐based predictive model were used to investigate the ecological processes relating to the formation and growth of colonies (local populations) of Audouin’s Gull (Larus audouinii Payraudeau). A colony was established in 1981 at the Ebro Delta (in the north‐western Mediterranean) and since then, the colony has grown dramatically at an average rate of 44% per year, such that by 1997 it held 65% of the total world population of this species. Our model shows that the observed growth in the colony cannot be explained without large‐scale immigration of individuals into the colony. Although immigration occurred in most of the years throughout the study, years with an absence of immigration and even with net emigration from the colony also occurred. The most likely source of immigrants was the colony on the Chafarinas Islands (655 km southwards from the Ebro Delta), which has been the largest known colony excluding the Delta in every year since 1981. However, immigrants from other neighbouring colonies apart from the Chafarinas, and even from other colonies beyond the western Mediterranean metapopulation were presumably also involved. Immigration from other colonies is likely to be driven by temporal heterogeneity in the relative quality of the different colonies, combined with individuals dispersing to search for and select the most suitable breeding place, in order to increase their fitness. However, breeding success at the Ebro Delta colony was not autocorrelated from one year to the next, suggesting that the predictability of the environment was low. Predictability was also low in other colonies within the metapopulation, independently of their size or relative quality. Results suggest that the presence of conspecifics (rather than local reproductive success) may be more important for immigrants in breeding site selection. Colonization of the Ebro Delta was probably facilitated by the effective protection of the site against human disturbances, which produced a new, empty, high quality site for prospecting gulls. This may be considered a stochastic and density independent event, of a type that may have been a much more important influence on seabird colony formation than has been appreciated.