Régulation par le recrutement chez la mouette rieuse Larus ridibundus

The Black-headed Gull population of the Forez (central France), under study since 1976, is remarkably stable. This stability, or slow growth at the very most, implies a change in some demographic parameters of the population. Indeed, large differences in breeding success were found between colonies established on traditional sites, successfully occupied year after year (where breeding success is high), and those established on sites that are occupied discontinuously (where breeding success is low). Such differences suggest that the gull population has reached saturation at the best sites. Most of our work was carried out in one of the traditional colonies (with more than 2 000 breeding pairs). Here, the saturation hypothesis is supported by the strong competition taking place for nest sites, which leads the younger breeders to make use of nest sites low over water, therefore highly vulnerable to wave action, and to variations of water level. A study of mortality and recruitment, using a capture-resighting technique, provided a more direct evidence of population saturation on this colony : full breeding was only reached at five years of age. But the strongest evidence of density-dependence in recruitment is brought forward by the estimation of the proportion of birds locally recruited is each cohort : this proportion varies inversely with the initial fecundity of the cohort. Only a limited number of breeders can therefore be recruited each year in this colony. A density-dependent population model confirms that density-dependence is strong enough to induce a stabilization in numbers of the colony members. Theoretical and practical consequences of this mode of population regulation are discussed.Lebreton Jean-dominique. Régulation par le recrutement chez la mouette rieuse Larus ridibundus . In: Revue d'Écologie. Supplément n°4, 1987. pp. 173-182