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

1. Simulations of a stochastic, individual-based predictive model were used to investigate the ecological processes relating to the formation and grow th of colonies (local populations) of Audouin's Gull (Larus audouinii Payra udeau). A colony was established in 1981 at the Ebro Delta (in the north-we stern Mediterranean) and since then, the colony has grown dramatically at a n average rate of 44% per year, such that by 1997 it held 65% of the total world population of this species. 2. Our model shows that the observed growth in the colony cannot be explain ed 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 als o occurred. 3. The most likely source of immigrants was the colony on the Chafarinas Is lands (655 km southwards from the Ebro Delta), which has been the largest k nown colony excluding the Delta in every year since 1981. However, immigran ts from other neighbouring colonies apart from the Chafarinas, and even fro m other colonies beyond the western Mediterranean metapopulation were presu mably also involved. 4. Immigration from other colonies is likely to be driven by temporal heter ogeneity in the relative quality of the different colonies, combined with i ndividuals dispersing to search for and select the most suitable breeding p lace, in order to increase their fitness. However, breeding success at the Ebro Delta colony was not autocorrelated from one year to the next, suggest ing that the predictability of the environment was low. 5. 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 mo re important for immigrants in breeding site selection. 6. Colonization of the Ebro Delta was probably facilitated by the effective protection of the site against human disturbances, which produced a new, e mpty, high quality site for prospecting gulls. This may be considered a sto chastic and density independent event, of a type that may have been a much more important influence on seabird colony formation than has been apprecia ted.