Requirements for a Mixed Reproductive Strategy in Avian Species

Trivers (1972) suggested it would be adaptive for males of species that invest considerably in parental care to pursue a mixed behavioral strategy of helping a single female raise offspring while taking advantage of opportunities to fertilize eggs of other females. A working assumption of influential studies since Trivers has been that, in avian species, males forcefully fertilize females, resulting in a successful mixed strategy for the males. In many avian species it appears that males are unable to forcefully fertilize females. Therefore, the male mixed behavioral strategy requires a complementary female mixed strategy. Females that might willingly copulate with nonmate males include paired females involved in sophisticated cuckolds, newly bonded females, unpaired females attempting to obtain mates, or unpaired females attempting to reproduce with the help of nonmates. We studied male and female behavior in our marked study population of herring gulls to determine if both sexes pursue mixed behavioral strategies. Thirty-five percent of paired herring gull males copulated with willing nonmate females, but these females were unpaired marked females that solicited from the males. Presumably, the paired males also would have copulated with nonmate paired females if they had had the opportunity. Paired females did not attempt to copulate with nonmate males despite ample opportunity. Differences between the behavior of paired and unpaired females suggests that females pursue a mixed behavioral strategy depending on their pairing status. This difference between paired and unpaired females' behavior illustrates confidence of paternity. Parental effort provided to a female by a male is mutually dependent upon confidence of paternity. This relationship has generally been ignored by influential studies of MRS that focus on EPC attempts and mate guarding, the inferred consequence of successful EPC. We found that male herring gulls increased their time on the territory during the fertile period. Although increased attendance could be used to infer that mate guarding was necessary, other data, including no increase in intrusions and no change in males' aggressiveness during the fertile period, and the fact that females were unattended for 20.2% of the fertile period, suggest that mate guarding may be relatively unimportant in herring gulls. Males were able to leave mates alone because of confidence of paternity provided by the female in exchange for parental effort in the form of courtship feeding. If the male EPC behavior in our study, and in other studies, is adaptive (i.e., increases the reproductive success of participating males and females) there must be situations in which EPC leads to successful rearing of offspring. Studies of female behavior will permit discovery of these situations.