Great Frigatebird Kleptoparasitism: Sex-Specific Host Choice and Age-Related Proficiency

Avian kleptoparasitism is a specialized foraging behavior requiring rapid flight speeds and extreme agility. As such, it is expected that proficiency will correlate with experience and that the size of the kleptoparasite will influence its choice of host species. I investigated age- and sex-differences in the kleptoparasitic behavior of Great Frigatebirds (Fregata minor) on Midway Atoll. The results indicate that chases by more than one frigatebird are more successful than solitary ones, and that success rates vary with the host pursued. In contrast with previous work, success improved with age and sub-adults were more likely to chase as part of a group than were adults. Adults showed striking sex-differences in their selection of host species, females preferring large species such as boobies and tropicbirds while males preferred the smaller noddies and terns. The role of these sex-differences in the evolution of sexual size dimorphism is discussed. Finally, since marked breeding individuals were not observed in kleptoparasitic events near the colony, this foraging mode is apparently not important to breeding birds in this population. However, in some cases the frigatebird kleptoparasitism may directly influence the behavioral patterns and the survival of host individuals.