Habitat structure and colony structure constrain extrapair paternity in a colonial bird

Individual variation in sexual fidelity and extrapair paternity (EPP) is widely attributed to environmental heterogeneity, but the only variables known to be influential are food abundance and density of conspecific breeders (potential extrapair partners). Habitat structure is thought to impact EPP but is rarely measured and, when considered, is usually confounded with food abundance and predation pressure. To sidestep these confounds, we tested whether EPP is associated with habitat structure variables and with local conspecific density in a species whose nesting habitat is not used for feeding and lacks predators. In a blue-footed booby, Sula nebouxii, colony, the probability of EPP in a female's nest was highest in parts of the study plot where there were few obstacles to locomotion, and was quadratically related to local density of sexually active males, even though local males did not sire the EP chicks. The probability of a male breeder siring EP (extrapair) chicks elsewhere was quadratically related to local density of sexually active males around his nest. From these patterns we infer that both sexes may foray for EP interactions, that males and females nesting at intermediate density are most likely to be accessed by forayers, and that obstacles in the vicinity of a female's nest constrain access of foraying males. To our knowledge, this is the first demonstration that individual variation in EPP is associated with habitat structure in the absence of confounding variation in food availability, predation pressure or breeder quality, and the first evidence that EPP opportunities of female and male breeders are reduced by high density of conspecific breeders above a particular threshold.