Pirates of the air and sea, but bedfellows too

Part of the frigatebird roost on Bird Island
Part of the frigatebird roost on Bird Island (Chris Feare)

Frigatebirds, those aerial giants of tropical waters, once called man-o-war birds by seafarers, are renowned for their aerobatic prowess when chasing other birds, especially boobies, in order to persuade the latter to regurgitate their last meal, which the frigatebirds eagerly devour. However, on Aldabra and elsewhere, frigatebirds and boobies breed alongside each other and live apparently amicably.

Bird Island hosts a huge number of breeding seabirds, especially during the south-east trade wind season from May to October when hundreds of thousands of Sooty Terns and thousands of Brown Noddies and Lesser Noddies nest in the varied habitats offered by the island. Frigatebirds do not breed but up to 500, mainly Lesser Frigatebirds, roost at night in tall Casuarina trees at the southern end of the Sooty Tern colony. Now in July 2016 they have been joined by at least 80 Red-footed Boobies and numbers of these seem to be increasing. The boobies are mainly young birds; a few roost singly but most roost in tight huddles of ten or more birds. They generally choose dead branches for their roosting sites and are surrounded by frigatebirds.

Red-footed Booby and frigatebird discussing relationshps in the Bird island roost
Red-footed Booby and frigatebird discussing relationshps in the Bird island roost (Chris Feare)

As early as 1973 I noticed injured and dead adult Sooty Terns on Bird Island’s western beach, with broken or dislocated wings, and assumed that they had been injured while drinking close to the shore, either by colliding with waves or with each other. Throughout the 1990s and 2000s injured birds were found each year and in 2014 we decided to investigate further, having begun to doubt these causes.

 

A Sooty Tern with a dislocated carpal joint stranded on thee beach
A Sooty Tern with a dislocated carpal joint stranded on the beach (Chris Feare)

Christine and I, along with colleagues from the University of Reunion, Camille Lebarbenchon and Muriel Dietrich, found that most of the dead and injured birds appeared on the beach in the morning rather than in the afternoon; flights to the nearby sea to drink take place in the hotter parts of the day and if Sooty Terns

Sooty Terns with wing broken and humerus bone protruding through the skin (Chris Feare)
Sooty Terns with wing broken and humerus bone protruding through the skin (Chris Feare)

were injured then we would expect the injured birds to arrive on the beach in the late afternoon or evening. We observed that frigatebirds harried Sooty Terns, Brown and Lesser Noddies, especially in the evenings. This is when foraging Sooty Terns normally return to the colony bringing food for their chicks and we observed them being grasped by frigatebirds, both by the wings and tail. Noddies were grasped much less often and Lesser Noddies in particular seemed to disgorge their fish soon after a frigatebird began its attack.

When we caught and handled Sooty Terns and both species of Noddy we found that while Noddies readily disgorged food, Sooty Terns were more reluctant. If the responses to being chased and grasped by frigatebirds are similar, the reluctance of Sooty Terns to regurgitate could account for their rougher treatment by frigatebirds, since while we found 84 injured Sooty Terns during our study, we encountered only one injured Lesser Noddy and only three Brown Noddies on the beach.

We concluded that frigatebirds were responsible for the Sooty Terns that we found dead and injured above the tideline, and three times in the sea near the shore. We also thought that the injured birds that managed to reach the beach must be only a small proportion of the number that are ultimately killed by frigatebirds, since most are likely to die at sea and never be found by us.

Despite the potential threat to Sooty Terns returning with food, they do not harass frigatebirds over the colony or at sea, and both can be seen soaring over Bird Island together in the strong south-east trade winds.

For the full story see:

FEARE, C.J., DIETRICH, M., LAROSE, C.S. & LEBARBENCHON, C. 2015. Injuries sustained by beached adult Sooty Terns Onychoprion fuscatus on Bird Island, Seychelles, during the breeding season. Marine Ornithology 43: 173–177.

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