Kleptoparasitic Behavior of the Magnificent Frigatebird: Sex Bias and Success

Abstract

Kleptoparasitism (the stealing of food) by the Magnificent Frigatebird (Fregata magnificens) was recorded over three breeding seasons at Isla Isabel, Nayarit, M6xico. Observation of chases on the principal target, the Blue-footed Booby (Sula nebouxii) were made in front of their joint breeding colonies. Most attacks were conducted by female and juvenile frigatebirds; male frigatebirds were not kleptoparasitic near their breeding colony. The proportion of chases provoking the target to regurgitate (success rate) was low (0.059) and only on 67% of those cases did the frigatebird get any food. Attacks were more frequent in the afternoon, when the boobies were returning to their breeding colony and were likely to be transporting food. In addition, frigatebirds may be evaluating the target's profitability in short chases, since longer chases provided a proportionately higher success. If so, the success reported above underestimates the profitability of these behaviors because it considers all chases equally. Group pursuits were three times more successful than those performed by single individuals. We did not find differences in the success and sexual bias in the kleptoparasitic tendencies between winter (frigatebirds' courtship and laying season) and spring (frigatebirds' feeding season) observations.