Competition for Nesting Space among Peruvian Guano Birds

Abstract

Abstract In the mid-1940's, Peruvian managers greatly increased the nesting space available to the three principal surface-nesting species of the Peruvian Coastal Current: the Guanay Cormorant (Phalacrocorax bougainvillii), the Peruvian Booby (Sula variegata), and the Peruvian Brown Pelican (Pelecanus occidentalis thagus). The combined populations of these three species increased from 8 to 20 million birds. The annual rate of increase of the population rose from 8 to 18%. The three species appear to have evolved in the face of a shortage of nesting space. They have not diverged in their respective breeding seasons. Each species has habitat preferences for nesting, but the overlap is great. The booby and cormorant compete through a "scramble" to occupy space before it is settled by the other species. Neither can displace the other from nest sites. The pelican is dominant over the other two in aggressive interactions and frequently usurps their nests. Pelicans are apparently confined to nesting in level areas, whereas the other two species can nest on a greater range of gradients. Despite the facts that nesting space is limited and that its scarcity has a demonstrated effect upon the combined populations, interspecific competition for nesting space was difficult to document. Interspecific aggressive interactions were few and involved only a small percentage of the three populations. The individuals most affected by competition, those denied nesting space, were displaced from the area of competition and were thus less accessible for study.