UC Davis

Reproduction is perhaps one of the most energetically expensive activities that an individual performs in a lifetime. Many species have evolved behaviors and tactics to offset the energetic and physiological costs of reproduction while optimizing long-term reproductive success and survival. Some individuals do so by parasitizing the reproductive efforts of other individuals, thereby reducing the costs of reproduction while still maintaining some reproductive output. Conspecific brood parasitism (CBP) is one such alternative reproductive tactic whereby some females lay eggs in the nests of females of the same species to avoid the cost of rearing and caring for those offspring. The evolution of this behavior has received considerable theoretical attention, but surprisingly there have been few quantitative studies examining the interactions between parasitic females and the host of the nest. We used motion sensor cameras and radio frequency identification devices to record the behavior interactions at the nest among female wood ducks (Aix sponsa) in a population in California. Our observations revealed a remarkable range of responses in female-female interactions. We classified females as “owners” if they incubated the nest and as “intruders” if they entered the nest, potentially laid eggs but did not incubate the nest. There were significant differences in behavior and variation of response for owners and intruders; intruders typically exhibited passive, nonaggressive behaviors and avoided interactions with the owner, while owners often behaved aggressively, although they also showed a wider variation in response. Repeatability (measured as the interclass correlation coefficient) of female behavior was significant and high for owners, but low among intruders. We tested several hypotheses on the possible factors that might account for variation in responsiveness, including the effects of status, age/experience, body size/phenotype/resource holding potential, history with the nest/previous ownership, and nest site timing. Status had a strong influence on all behavioral dynamics. Absolute age and body weight/condition did not influence the level of response, but relative differences in age and body weight/condition between owners and intruders had a strong effect. Owners were increasingly more aggressive when they were relatively older or larger than a given intruder. However, these same relative differences in age and size did not affect the behavioral response of intruders. Owners were also more aggressive to intruders during incubation than during the egg-laying period. There was no effect of previous history of the female or use of the box. Intruders laid an egg during 37 of the 57 observed encounters with owners, indicating that the intruders were, in at least most cases, acting as brood parasites. Owner aggressiveness did not prevent intruders from laying an egg. The passive avoidance of intruders in all interactions suggests strongly that these were parasitic interactions and not simply instances of two females fighting for possession of a nest site. The variation in the observed level of aggressiveness among pairs of females is intriguing and suggest that owners may act differently to some females; future research will explore the pattern of kinship and social relationships among these females.