Temporary male removal increases extra-pair paternity in eastern bluebirds

The effects of temporary male absence during egg laying on the production of extra-pair young by female eastern bluebirds,Sialia sialis, were examined. Females whose mates were removed for 2 days during egg laying produced more extra-pair young than did females whose mates were present during egg laying. Replacement males were observed at three of 11 nests from which resident males were temporarily removed, and one of 13 extra-pair young was sired by a replacement male. The increased production of extra-pair young when males were removed during egg laying suggests that (1) faced with apparent mate loss, females used copulations in an attempt to attract replacement males to care for young, (2) females were released from the constraints of mate guarding and sought extra-pair copulations, or (3) absence during egg laying reduced the ability of males to devalue rival sperm via supplemental copulations.