TERNS Sternidae
  • Species in family   44
  • Species observed [DR]  34 (77%)
  • Species photo'd [DR]  27
Terns are a distinctive group of arial fish feeders. They are closely related to gulls (Laridae) but are more specialized in terms of nesting habitat, diet and foraging methods, and their morphology reflects these specializations (Gochfeld & Burger 1996). Almost two-thirds of the species are "typical" terns in the genus Sterna, represented here by the mid-sized Elegant Tern (group top of page) of the Pacific Coast of California and west Mexico. It is one of six species of "crested" terns found along tropical and temperate coastlines around the globe. All the "crested terns" nest in very dense colonies with incubating adults just a body length apart. Despite what you may see in field guides, there is quite a bit of variation in bill (and leg) color in young Elegant Terns, as can be seen in the photo. This species colonized southern California within the last half-century and is now common in summer and fall north to Monterey Bay.

Another group are a half-dozen "noddies & relatives" in three genera that are restricted to tropical oceans. Among them is the beautiful White Tern (left or above left), shown here at its nest site on the island of Cousin in the Seychelles. The adult dozes near a just-hatched chick. These terns make no nest but lay a single egg in the fork of tree branch and incubate it there until the fluffy baby hatches.

Terns are closely related to gulls and skimmers; indeed, much recent literature follows Sibley & Monroe (1990) in considering them simply a tribe (not even a subfamily) of an expanded Laridae which includes gulls, terns, jaegers, and skimmers. I prefer the traditional approach that considers terns to constitute a family, an approach used in the on-going Handbook of the Birds of the World series (Gochfeld & Burger 1996). This approach emphasizes their distinctiveness while recognizing that they evolved from larid-like ancestors. 

While about half of the species of terns in the world are primarily birds of tropical oceans and coastlines, small Sterna terns occur in a wide range of habitats. Some are indeed tropical, like this White-cheeked Tern (right) of the western Indian Ocean, ranging north into the Red Sea and Persian Gulf. In basic plumage (as shown here) many of the species are very similar. All of these species have comparatively similar body shapes evolved to dive for fish from the air. All have swift and graceful flight styles.

There are huge differences in the ranges of these small-sized Sterna terns. Common Tern S. hirundo is a widespread breeder in the Northern Hemisphere and then migrates in huge numbers to the Southern Hemisphere in winter. Arctic Tern S. paradisaea is renowned for its exceptionally long migration route from the high Arctic to Antarctic winter grounds. There are other migratory species that nest around the Bering Sea (Aleutian Tern S. aleutica) and on the Antarctic Peninsula (Antarctic Tern S. vittata). Many of these are mostly pelagic species but others (e.g., Little Tern S. albifrons in the Old World, Least Tern S. antillarum in the New World) follow rivers and wetlands to nest well inland. 

A very unique species that forages in the cold Peru Current off western South America is Inca Tern (below left). It is generally placed in its own genus (Larosterna) because of its remarkable plumage and because of its distinctive breeding behavior. It nests in deep crevices and small caves along the arid coastline; I photographed the one shown here with a flash inside one of those small caves in coastal Peru.
In contrast to the limited range of Inca Tern, endemic to the Peru Current, the range of Sooty Tern (above right) is very broad. It is found in all the tropical oceans of the world. I think that Sooty Tern is one of the most remarkable birds in the world. Except when adults come ashore to nest, the entire world population spends its entire life in the air over tropical seas. Its plumage is not water repellent, so it does not land on water. Rather, it is constantly in the air for months and perhaps years at a time, sleeping in short snatches on the wing. The subadult plumage shown here is almost never seen near land and is therefore little known; I was lucky enough to spent four months at sea in the eastern tropical Pacific and got to know this bird. Beyond its amazing habit of constant flight, this is also the species on which other tropical seabirds key. Foraging Sooty Terns are the first to find the tuna-dolphin associations foraging on small fish (Au & Pitman 1996). At sea in the tropical Pacific, a cloud of Sooty Terns hovering in the distance usually means there is a foraging aggregation of tuna, various tropical dolphins, and many seabirds. Yet, as the ship approaches, the terns are the first to depart. All in all, a most impressive tropical seabird. Because of its unique distribution, it can get caught up in hurricanes and typhoons and end up far from it normal habitat (e.g., records north to Japan, California, and the northeastern U.S.).

A few terns are mostly interior waterbirds, and among them are the three "marsh terns" in the genus Chlidonias. They, too, are highly migratory. Species that migrate long distances can produce vagrants far from normal migration routes. One example is this lovely breeding-plumaged White-winged Tern (below left). It foraged over the Arcata wastewater ponds for several weeks in summer 1996 where it represented the first record for California and, indeed, the first record for the western United States. Three years later a one-year-old White-winged Tern appeared on Elkhorn Slough in my home county (Monterey Co.) for the second state record. It also stayed over a month, and was thus observed by hundreds of eager birders.
Most terns nest in colonies, although only a few species have the tightly packed colonies of the "crested terns." Caspian Tern (an almost-ready-to-fly youngster is shown above right) is a worldwide species that nests locally in loose colonies. It is an important 'core' species of tern and skimmer colonies in California because the large Caspian Terns can and do keep predatory gulls from eating eggs and chicks. The endangered California race (browni) of Least Tern, for example, succeeds only where it can nest around the fringes of a Caspian Tern colony. As can be seen in the photo (above right), juvenal plumage can be very striking and beautiful. It helps protect youngsters while on the ground, but they soon molt into a more uniform pale gray-backed plumage once they are old enough to fly.
Although most of the world's terns are white or pale gray or both, there are dark brown or blackish colored species, including Inca Tern (featured above). Best known, though, are the three species of "noddy terns" (genus Anous) that forage offshore in tropical oceans. They breed in large concentrations; American birders are very familiar with the huge Brown Noddy A. stolidus colony in the Dry Tortugas, Florida. A species that prefers to nest colonially in trees and small bushes on tiny islands is Black Noddy (left). Shown here are just two of several dozen Black Noddies that had just returned in the late afternoon to Heron Island on the Great Barrier Reef. They had been fishing all day offshore and now were drying their wings before flying up into their nest trees. The bungalows here are scattered among the Casuarina trees used by the nesting noddies, and the din of them squabbling all night long is incredible.
Photos: The flock of Elegant Terns  Sterna elegans was at the Salinas River mouth, Monterey Co., California, on 11 Sep 1994. The adult and baby White Tern  Gygis alba were at a nest site on Cousin I., Seychelles, in Nov 1992.  The basic plumaged White-cheeked Tern Sterna repressa was following the incoming fishing fleet at Fujairah, United Arab Emirates, in March 2001. The adult Inca Tern Larosterna inca was at islets off the Paracas Peninsula, Peru, in June 1986. The subadult Sooty Tern  Sterna fuscata was thousands of miles offshore in the eastern tropical Pacific in Oct 1989. The adult White-winged Tern  Chlidonias leucopterus was California's first record at Arcata marsh, Humboldt Co., on 25 June 1996. The fledgling Caspian Tern  Sterna caspia was at the breeding colony at the Salinas R. mouth, Monterey Co., California, on 17 Jun 2004. The drying adult Black Noddies Anous minutus were on Heron I., Great Barrier Reef, Australia, on 25 Nov 1983. All photos © 2004 Don Roberson; all rights reserved.

Bibliographic note

There is no "family book" per se although terns are included in numerous texts which cover gulls and relatives. The account by Gochfeld & Burger (1996) in the Handbook of the Birds of the World series is an excellent introduction. There are several recent books on tern identification but none covers all the species. I found Olsen & Larsson (1995) to be quite good for the 23 species that it covered although it seemed better for the Old World than the New. I understand that there is another tern i.d. book of which the late Claudia Wilds was a co-author. I have not yet seen it, nor have I found it yet available for sale. It should be quite good, though, given Claudia's excellence in all things having to do with identification.

Literature cited:

Au, D.W.K., and R.L. Pitman. 1986. Seabird interactions with dolphin and tuna in the eastern tropical Pacific. Condor 88: 304-317.

Gochfeld, M., and J. Burger. 1996. Family Sternidae (Terns). Pp. 624-667 in del Hoyo, J., Elliott, A., & Sargatal, J., eds. Handbook of the Birds of the World. Vol. 3. Lynx Edicions, Barcelona.

Olsen, K.M., and H. Larsson. 1995. Terns of Europe and North America. Princeton Univ. Press, Princeton, N.J.

Sibley, C.G., and B.L. Monroe, Jr. 1990. Distribution and Taxonomy of the Birds of the World. Yale Univ. Press, New Haven, CT.




Page created 30 Oct 2004