Communal Breeding in Tropical Guira Cuckoos Guira guira: Sociality in the Absence of a Saturated Habitat

Abstract

The evolution of sociality in many communally breeding birds has been ascribed to a lack of optimum habitat, that is, insufficient high quality resources to allow dispersal and independent breeding of young. In these species, offspring do not disperse immediately, but instead remain as helpers in their natal territory until a breeding opportunity occurs. We examined the habitat saturation model for a communally breeding South American species, the Guira Cuckoo. This species breeds in large groups composed of both related and unrelated individuals. Nestlings also exhibit varying degrees of relatedness, or they may be completely unrelated. Thirteen ecological variables were measured in 14 sites occupied by groups and compared with data from nine vacant sites that appeared similar. Data concerning breeding traits were collected for each reproductive event. Guira Cuckoos exhibit tactics that reduce the reproductive success of others within the group, mainly egg-tossing behavior and, occasionally, infanticide. Potential nesting sites within territories, in the form of araucaria trees, varied greatly in number, but there was no relation between their availability and the size of the group controlling the area. Group size was significantly positively correlated with grass cover within territories, but unrelated to the other 12 vegetation variables measured. Empty sites had a significantly greater abundance of prey items than did areas occupied by Guira Cuckoos, which may indicate depletion of insects due to the cuckoos' foraging activities. No other significant differences were found between occupied and empty sites. These results suggest that habitat saturation may not be an important factor in the maintenance of group-living in Guira Cuckoos. However, this does not exclude the possibility that habitat quality may have an important effect on reproduction. The basis for group-living in this species is most likely related to intrinsic characteristics of sociality that increase survival and lifetime reproductive fitness.