Population growth, range expansion, and competition in the cooperatively breeding brown jay, Cyanocorax morio

Abstract

Abstract. Observations of an expanding population of brown jays in Monteverde, Costa Rica made it possible to characterize a transition from incomplete habitat saturation, in which jays maintained moderate group sizes, to more complete habitat saturation, in which jays maintained larger group sizes and showed increased aggression at high densities. From 1977 to 1990, the number of individuals, number of flocks, and mean flock size on the study area increased dramatically. Only at high densities did flocks comprised of fewer than six individuals attempt breeding, even though suitable habitat (indicated by later successful breeding there), had been available for the past decade. The population has expanded its local range as well, and now occupies all of the preferred cleared habitat in the study area. Coincident with these demographic changes, the emergence of overt intra-group aggression at the nest was observed, especially that involving interference competition between rival older females during the building/laying stage of the nesting cycle. Aggressive interactions at a nest were much more likely in flocks with more than 10 individuals and also more likely as the number of birds seen sitting on the nest increased. The positive relationship between number of offspring produced and the number of old birds in a flock was weakened at higher densities, probably resulting from increased intra-group aggression. These observations suggest that high population density and limited breeding opportunities resulted in increased reproductive competition. Sociality, however, has not simply been forced in this population, because even at low densities in the presence of ample unused habitat Monteverde brown jays bred only in groups. This suggests that intrinsic benefits to group living probably have also played an important part in maintaining sociality in this population.