Adaptive value of thermally inefficient male incubation in Barn Swallows (Hirundo rustica)

Most male passerines do not develop brood patches. Despite this, some, including Barn Swallows (Hirundo rustica), apparently incubate. Male incubation should stabilize and enhance average egg temperature over the course of embryonic development and allow females more time to forage before eggs cool to suboptimal temperatures. We recorded egg temperatures and monitored incubation behavior in three Barn Swallow colonies. Female incubation and male nest attendance produced higher average egg temperatures than periods of unattended egg cooling. Females reheated eggs at significantly higher rates than males. Eggs normally cooled during male on-bouts and when unattended, though male nest attendance slowed cooling significantly. Incubation cycles with male nest attendance had less thermal variation than those that lacked male assistance. Embryonic development time was not correlated with male nest attendance. Although male nest attendance is unlikely to enhance embryonic development, it appears to play a significant role in enabling females to forage longer when off nests. Our observations suggest that male incubation in Barn Swallows most likely serves to reduce the probability of nest desertion by females and to increase the male's reproductive output.