Use of an Honest Signal by Red-footed Boobies <i>Sula sula<i> in Response to Kleptoparasitism by Great Frigatebirds <i>Fregata minor</i>

On Genovesa Island (00.3°N, 089.9°W) in the Galápagos Islands, Ecuador, Great Frigatebirds Fregata minor kleptoparasitize (i.e., steal food from) Red-footed Boobies Sula sula. In five of eight cases that I observed, the frigatebirds harassed the boobies until the latter regurgitated their forage, which the frigatebirds consumed. In three cases, the booby responded with a honk-like call and the frigatebirds stopped harassing. I propose that the honk is an “honest signal” by a booby having little food to divulge. Boobies loaded with forage do not honk because the call could trigger regurgitation. Thus, frigatebirds harass with escalation only non-honking boobies. The interaction is important in what appears to be a food-limited booby population.