The piratical behaviour of frigatebirds, involving chasing other seabirds until these targets disgorge their latest meals for the frigates to snap up, is well known and led sailors to refer to them as Man-o’-war birds.
Large numbers of frigatebirds, mainly Lesser Frigatebirds, roost at night in tall Casuarina trees on Bird Island and many also roost by day, often posing with wings outstretched and twisted in such a way as to expose the undersides of the wings to the sun.
Frigatebirds can feed themselves, catching flying fish and other species that jump out of the water but their feathers are poorly waterproofed and they cannot settle on the water or dive. In some places, e.g. Aldabra, they also take turtle hatchlings and the Ascension Frigatebird, an endemic species on that Atlantic island, even takes recently hatched Sooty Terns that have been left unattended by their parents. Frigatebirds are also adept pirates and boobies are particularly vulnerable to attack. Smaller seabird species are also persuaded to regurgitate their hard-won gains, however, and we have often witnessed this off Bird Island.
In the 1970s I noted that one of the commonest sources of mortality of adult Sooty Terns at the colony was of birds found on the beach with broken or dislocated wings, some of them already dead but living birds presenting a sad sight with no hope of survival. I attributed these birds to individuals that had collided in mid-air in the melee of birds flying over the colony, or to individuals that had mis-timed flights to drink sea water, a commonly seen behaviour close to the island, and had collided with waves.
In 2014, Christine Larose and I, with help from our University of Reunion colleagues Muriel Dietrich and Camille Lebarbenchon, studied these dead and dying Sooty terns in more detail. Collision with other birds or with waves proved to be highly implausible explanations for the injuries and we began to suspect frigatebirds. Frigatebirds chased birds most actively in the late afternoons when Sooty Terns were returning with their day’s catch. By recording the time that injured birds were found on the shore, and their locations on the beach, we established that most were found in the mornings and at lower levels on the beach, with a few being found in the water’s edge. This is what we would expect if birds were injured by frigatebirds in the evening and had to swim ashore.
Watching frigatebirds close offshore revealed that in addition to Sooty Terns, they also chased Lesser, and to a smaller extent Brown, Noddies. However, the aggressiveness involved in the chases varied between the species. Lesser Noddies were merely chased, whereas Brown Noddies, and especially Sooty Terns, were more frequently chased down to the water and often grasped by a wing or the tail. While we were handing Lesser Noddies and Sooty Terns we found that Lesser Noddies were much more likely to give up their last meal than were Sooty Terns. This greater reluctance of Sooty Terns to regurgitate could explain the frigatebirds’ rougher treatment of them and explain why we found many injured Sooty Terns but few injured Lesser Noddies.
During the 2014 Sooty Tern breeding season we found 84 beached Sooty Terns on Bird Island. However, it seems likely that many injured birds would not make it to land and would die at sea or even be eaten by fish. The birds we found might therefore represent a small proportion of the birds killed each breeding season by frigatebirds. Man-o’-war birds could thus be a significant source of adult mortality in the Bird Island Sooty Tern colony.
The results of this study will be published shortly: Feare, C.J., Dietrich, M., Larose, C.S. & Lebarbenchon, C. 2015. Injuries sustained by beached adult Sooty Terns Onychoprion fuscatus during the breeding season. Marine Ornithology 43: 173-177.