AbstractThe frequency with which individual male purple martins (Hirundinidae: Progne subis) guard their mates during forays to gather nest material varies from 0 to 100%. During such forays, guarding males accompany their mates continuously from the time a female leaves the nest site until she returns to the nest site with nest material. Non-guarding males do not accompany females on these collecting forays. Unguarded females are subject to forced extrapair copulation attempts on almost all of their forays to collect nest material. Because this suggested that non-guarding males may have a lowered assurance of paternity relative to guarding males, I asked if mate guarding frequency is directly related to parental care. However, male parental effort, as assessed by feeding rates, was not related to mate guarding intensity. Mate guarding intensity may vary widely among individual male martins since nest site defense may preclude guarding in some and superiority in sperm competition by means other than exclusive access to females may reduce their chances of being cuckolded.