FEMALE-DEFENSE POLYGYNY IN A NEOTROPICAL BIRD, THE MONTEZUMA-OROPENDOLA

In a Costa Rican population of the Montezuma oropendola, Psarocolius montezuma, males defended groups of sexually receptive females. Females nested colonially, and males aggressively competed with each other for access to groups of females at nesting colonies. Males were ranked based on the outcome of aggressive interactions. The top-ranking or alpha male physically excluded all lower-ranking males from the colony and prevented them from approaching females. The alpha male was present at the nesting colony during those periods of the day when the highest number of females was present. During other parts of the day, the alpha male was often absent and lower-ranking males moved into the colony and defended females there until the alpha male returned. Over the course of the season, the number of males present at the colony was positively correlated with the number of sexually receptive females there. Male competition was localized at that portion of the colony containing the highest number of nest-building females; when females finished their nests and laid eggs, males shifted to areas containing females that had not yet laid eggs. Males also moved to new colony sites whenever the females moved, both within and between breeding seasons. Male mating success at the focal observation colony was strongly and positively associated with dominance rank; 77% of 88 copulations observed were obtained by the alpha male. These results indicate that the Montezuma oropendola has a female-defence mating system. This mating system is more similar to those of polygynous mammals than it is to those of most other polygynous birds.