Evidence for Social Regulation in Some Riparian Bird Populations

Abstract

Patterns in the regulation of seven permanent resident riparian bird species were studied in the lower Colorado River valley. Populations may be predator-food regulated or selection might favor dispersal tendencies that keep the population at a given location low enough to avoid resource depletion as well as the attraction of predators. Theory states that socially regulated species should have relatively short breeding seasons and less postbreeding mortality, and should begin population declines later than nonsocially regulated species. Among the seven species studied we found a continuum, with ladder-backed and Gila wood-peckers in agreement with the theoretical criteria of socially regulated species and the cactus wren, verdin, and Gambel quail at the opposite end of the spectrum. The Abert towhee and crissal thrasher showed criteria of both groups, but had more in common with socially regulated species.