Seabird predation by an insular subspecies of the Common Buzzard Buteo buteo toyoshimai on Minamijima, Ogasawara Islands

Abstract

The insular subspecies of the Common Buzzard Buteo buteo toyoshimai, of the Ogasawara Islands, is extremely rare and is considered endangered. Recently the buzzard has been observed to prey on the introduced Black Rat Rattus rattus. Eradication of the rat, which has been proposed as a conservation measure for native island biota, may lead to a food shortage for the buzzard. In this study, I studied the native seabirds as candidate prey alternatives to the rat for the buzzard and clarified prey species, breeding stages of the prey, and predatory behavior of the buzzards. Carcasses of seabirds were examined and behavioral observations of the buzzard were conducted on Minamijima, part of the Chichijima island group. Of the 86 prey remains found, 66 carcasses were Wedge-tailed Shearwaters Puffinus pacificus and 16 were Bulwer's Petrels Bulweria bulwerii, which collectively accounted for 95.3% of the total. The preyed-upon carcasses of adult Wedge-tailed Shearwaters were found during April, May and June, those of adult Bulwer's Petrels were found in June and July, Bulwer's Petrel chicks of were found in September and October, and Wedge-tailed Shearwater chicks from September to January. Buzzards preyed on adults of both species from the egg laying to the brooding period and on almost grown nestlings. The larger Brown Booby Sula leucogaster appeared to be unlikely prey for the buzzard. The buzzards walked or flew to search for seabirds, and when found, used their feet to pull them from rock crevices. Adult Wedge-tailed Shearwaters may be too heavy for the buzzards to carry to their nest if caught far from the buzzards' nest. Moreover, Bulwer's Petrels are less likely to be fed to buzzard nestlings because Bulwer's Petrels return to their colony during the fledging season of the buzzard. For seabirds to function as prey, species with different breeding periods that will serve as alternative food sources will be necessary to compensate for the absence of Wedge-tailed Shearwaters and Bulwer's Petrels, while expanding the nesting sites of these species.