Coloniality in the Yellow-Rumped Cacique as a Defense against Nest Predators

Abstract

Abstract Individuals of the colonial Yellow-rumped Cacique (Cacicus cela) in Amazonian Peru can defend their nests against predators in three ways. First, by nesting on islands and around wasp nests, caciques are safe from arboreal mammals such as primates, which destroy many more-accessible colonies. Caimans and otters that live in lakes also protect island colonies from snakes, which are vulnerable when crossing open water. Second, by clustering nests together and mobbing as a group, caciques can deter many avian predators, which take spatially isolated nests in small colonies. The effectiveness of mobbing increases with group size, which in turn is correlated with colony size. Third, by mixing their enclosed, pouchlike nests with abandoned nests, caciques can hide their nests from some predators. Overall, nests in clusters on islands and around wasp nests suffer the least predation, largely because they are well protected against the cacique's major predators. Females switch colonies after losing nests to a predator, usually to sites that offer protection against that predator. By this mechanism, the best colony sites accumulate the largest numbers of nests. It is unclear, however, why all females do not nest in the safest colony sites. I argue that nest predation favors coloniality because of the scarcity of nest sites that are safe from mammals and the increased effectiveness of group defense.