POPULATION-GROWTH, RANGE EXPANSION, AND COMPETITION IN THE COOPERATIVELY BREEDING BROWN JAY, CYANOCORAX-MORIO

Observations of an expanding population of brown jays in Monteverde, C osta Rica made it possible to characterize a transition from incomplet e habitat saturation, in which jays maintained moderate group sizes, t o more complete habitat saturation, in which jays maintained larger gr oup 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, e ven though suitable habitat (indicated by later successful breeding th ere), had been available for the past decade. The population has expan ded its local range as well, and now occupies all of the preferred cle ared habitat in the study area. Coincident with these demographic chan ges, the emergence of overt intra-group aggression at the nest was obs erved, especially that involving interference competition between riva l older females during the building/laying stage of the nesting cycle. Aggressive interactions at a nest were much more likely in flocks wit h 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 w as weakened at higher densities, probably resulting from increased int ra-group aggression. These observations suggest that high population d ensity and limited breeding opportunities resulted in increased reprod uctive competition. Sociality, however, has not simply been forced in this population, because even at low densities in the presence of ampl e unused habitat Monteverde brown jays bred only in groups. This sugge sts that intrinsic benefits to group living probably have also played an important part in maintaining sociality in this population.