On Bird Island, Seychelles, adult Sooty Terns are frequently found injured on the beach, usually with dislocated or broken wings, during the breeding season. By ruling out other possibilities we hypothesized that the injuries were caused by frigatebirds, and therefore predicted that (1) most attacks would take place in the late afternoon, when Sooty Terns normally return to the colony after feeding during the day; (2) most injuries would be inflicted during the late afternoon and so injuried birds are most likely to be found on the beach the following morning, after they have swum ashore; and (3) frigatebird Fregata spp. attacks would be more frequent during chick rearing, when adults carry fish and/or squid for their chicks, than during incubation, when the carry only the food required for their own maintenance. At two-week intervals during the 2014 breeding season we undertook early morning and late evening 5-d surveys of the number of beached Sooty Terns, of frigatebirds chasing seabirds visible from the beach, and of frigatebirds in the communal roost. The first two predictions were supported by the data by the third was not; this failure was considered to be due to frigatebirds parasitizing other species, especially Lesser Noddies Anous tenuirostris, when these were more profitable sources of regurgitates. Overall, we conclude that frigatebirds are responsible for the injuries that cause Sooty Terns to be found on the beach, and that the number found on the beach is probably only a small proportion of the mortality inflicted on the Bird Island colony.
Keywords: Sooty Tern, Onychoprion fuscatus, frigatebird, Fregata spp., injury, Lesser Noddy, Anous tenuirostris, kleptoparasitism, Bird Island Seychelles