Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds

Approximately 45% of western bluebird (Sialia mexicana) females have some chicks in the nest that are not sired by their social mates. Extrapair fertilizations account for 42% of offspring in these nests and 19% of nestlings overall. I tested the hypothesis that males reduce nestling provisioning when their certainty of paternity or share of paternity is reduced. Capture and detention of socially monogamous males for 1 h or 24 h during the laying period reduced males' copulatory access and their ability to mate guard, increasing the frequency with which extrapair males intruded and attempted to copulate with resident females. Males detained during laying did not reduce their share of feeding trips compared to control males detained during incubation, compared to unmanipulated males, or compared to males that were captured but not detained. Males detained on territory for 1 h during the laying period did not reduce their share of feeding trips when they observed male intrusion, nor when they observed their mates accepting extrapair copulations. Males that witnessed their mates accepting extrapair copulations did not reduce their share of risk in provisioning. Genetic fingerprinting at nonexperimental nests indicated that males also failed to reduce their feeding contributions when their estimated share of paternity was reduced, even when a helper male was present to reduce the impact on nestlings. These results suggest that male western bluebirds do not make significant adjustments in their share of provisioning when they have evidence of partial paternity loss. Together with prior results, this study suggests that western bluebird males use an all-or-none rule, contributing approximately half of the parental provisioning at nests, as long they have some copulatory access to the female during egg laying. Copyright 2003.