Editor's Note: Study of the mitochondrial DNA of terns, along with their plumage characteristics, have suggested that the heretofore broadly defined genus Sterna is paraphyletic. Reclassification of this genus now places Gray-backed Tern in the genus Onychoprion. See the 47th Supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will account for this change.
The Gray-backed Tern is a denizen principally of the central Pacific Ocean. It breeds on small, remote islands and atolls, so its distribution and habits remain somewhat of a mystery. It is considerably shier and rarer than its close relative, the Sooty Tern (Onychoprion fuscatus), with which it often occurs and is sometimes confused.
In the Hawaiian Islands, the Gray-backed Tern's diet is highly complex. It feeds, by dipping and plunging, on a wide variety of fish, but it also eats crustaceans, mollusks, marine and terrestrial insects, cnidarians, and, elsewhere, skinks. Its favorite prey, the thornback cowfish (Lactoria fornasini), is consumed in great quantities. In fact, the Hawaiian name for the Gray-back, päkalakala (kalakala means “thorny”), may stem from this prey item. Although probably pelagic much of the year, this tern is thought to forage closer to land during the breeding season.
The Gray-back is migratory in the subtropical Hawaiian Islands, returning to breeding colonies in December and January, and departing again in August and September. Within colonies, nesting is not highly synchronous. Outside of the Hawaiian Islands, its breeding season is not well documented, but in the equatorial Phoenix Islands (3°S, 173°W), it may breed in any month of the year. Little is known of the Gray-back's breeding biology or behavior.
The Gray-backed Tern lays a single egg, deposited in a simple depression in the ground, and feeds its young by regurgitation. Reproductive performance is highly variable. Because the Gray-back tends to nest along island peripheries, its nests are often lost to high tides. Eggs are lost to predatory shorebirds, and chicks to marauding Great Frigatebirds (Fregata minor). Prebreeding flocks of Sooty Terns swirl over colonies, sometimes driving the more timid Gray-backs from nesting areas.
Knowledge of the Gray-back's historical distribution is clouded by inaccurate identifications and a dearth of information from many regions. Knowledge of its current distribution is similarly incomplete. If populations are to be protected, determination of the range of this tropical-Pacific endemic and creation of conservation areas are urgently needed. The gravest threat to this species is probably the introduction of mammals to nesting colonies, a widespread problem in Oceania.
The Gray-back has rarely been the focus of a major study. At Hawaiian breeding grounds, its food habits and requirements, energetics, physiology, and provisioning and growth of chicks have been investigated (Harrison et al. 1983, Ellis 1984a, Pettit et al. Pettit et al. 1984c, Pettit et al. 1984d, Pettit et al. 1985, Shea and Ricklefs Shea and Ricklefs 1985, Shea and Ricklefs 1996, Whittow et al. 1985). Much information about the species is anecdotal and can be found scattered in the literature and in unpublished reports, especially those of the Smithsonian Institution's Pacific Ocean Biological Survey Program (POBSP), which operated from 1962 to 1969, and of the U.S. Fish and Wildlife Service (USFWS).