The Sooty Tern is the world’s most numerous tropical seabird. Although it has been the subject of several major studies, two aspects of its life render investigation difficult. First, it generally breeds on remote tropical islands where access is often difficult. Second, it spends its entire life outside the breeding season at sea, living an almost entirely aerial existence as its poorly waterproofed feathers prohibit it from landing on water. Investigations of its movements and behaviour during this phase of its life cycle have been impossible until the recent advent of miniaturised data recording devices, which have facilitated glimpses of its marine life.
Despite difficulties of access to most breeding colonies, some of which can contain over a million pairs, many aspects of its breeding biology are well-known. However, the first episode of the BBC’s new David Attenborough extravaganza Blue Planet II, produced by the BBC Natural History Unit and broadcast on Sunday 29 October, opened our eyes with spectacular footage of a hitherto unknown aspect of Sooty Tern life in a breeding colony on Goelettes Island, an islet of Farquhar Atoll, one of the southernmost atolls of the Seychelles archipelago.
After hatching, Sooty Tern chicks take about two months to develop to the point of fledging. First flights are clumsy affairs, often resulting in crash landings a few metres from the nest site. Greater proficiency leads them to fly further but tiring or misjudgement can lead juveniles to land prematurely. If they do so on water, they have difficulty in taking off again.
In the lagoon on Farquhar Atoll, the BBC recordings showed the cost of such failures in flying ability – incapacitated juveniles floating on the water were prone to being eaten by Giant Trevallies Caranx ignobilis, large fish that are regarded as good sport by game fishermen. More remarkably, the fish were filmed leaping out of the water to catch juvenile Sooty Terns (and Brown Noddies) in flight low above the lagoon surface. This kind of fish predation has not previously been recorded for Sooty Terns but the discovery of the Giant Trevallies’ delectation for birds is also a first record of their inclusion in the diet of these fish.
From my personal point of view, this totally new facet of Sooty Tern biology was extraordinary but the entire first episode of Blue Planet II contained fascinating information on a wide range of marine life and threats that face it, illustrated by truly wonderful photography. I’m sure I am not alone in having my appetite whetted for the remainder of the series.