San Diego Natural History Museum--Your Nature Connection[San Diego County Bird Atlas Project]

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Royal and Elegant Terns

The salt works at the south end of San Diego Bay have become a resource of national importance to breeding birds, now recognized in their recent incorporation into the new San Diego National Wildlife Refuge. Among the many species of water birds nesting there, the Elegant Tern is among the most numerous. The Royal Tern also nests there, in much smaller numbers and perhaps only irregularly. The two species are similar in many ways, similar enough to raise a problem in identification. Their external differences are more in size and shape than in pattern of plumage, making an intimate familiarity with the birds and their habits all the more important in identifying them. Fortunately, here in San Diego we are in an ideal spot for gaining this familiarity.

Elegant Tern

Royal Tern

Illustrations by Nicole Perretta

The Royal Tern is a much bigger bird than the Elegant. It is actually closer in size to the Caspian. The difference in gross size is perhaps most easily seen in the birds' weights: 165 to 217 grams in the Elegant, 353 to 409 in the Royal, and 455 to 549 in the Caspian (figures from specimens in the San Diego Natural History Museum). Size, though, can be tricky to assess when the birds are in a single-species flock.

When the birds are sitting on the ground, I find the difference in bill shape the most obvious feature. Despite the Elegant's being a smaller bird, its bill is just as long as, even a hair longer than, the Royal's. But it is much thinner, measuring 11 to 12 millimeters thick at the base, versus 17 to 18 in the Royal. The Elegant's bill is about 5.7 times longer than thick, the Royal's 3.4 times longer than thick. Thus the Royal's bill shape is much closer to the Caspian's (3.2 times longer than thick). The Elegant's bill looks delicate and needle-pointed in comparison to the heavy daggerlike bills of the Royal and Caspian.

The more slender, "elegant" shape of the Elegant Tern carries through the wings as well. When the birds are in flight, I find this difference to be the most useful. Despite the Royal's being twice as bulky as the Elegant, its wings are only 12% to 15% longer. The result is that the Elegant Tern looks narrowly tapered to sharp points in all directions. The Royal Tern would be much more easily confused with the Caspian than the Elegant were it not for the obvious difference in plumage: forehead black or mottled in the Caspian, usually white in the Royal; undersides of primaries entirely dark in the Caspian, white with just an edge of dark in the Royal (and Elegant).

The two species are also easily distinguished by head pattern when in nonbreeding plumage. In the Elegant Tern, the black plumage of the nape extends forward to surround the eye completely and to cover the rear half of the crown. In the Royal, the black is much more limited, coming to a point behind the eye (there is also a small black spot in front of the eye). The crown is almost entirely white with just a black fringe and some inconspicuous black spotting on top. Thus the Royal could be dubbed the "male pattern baldness tern." The difference in head pattern makes distinguishing the species in the fall easy.

 

Elegant Tern

 

Royal Tern

Illustrations by Nicole Perretta

In the breeding season, though, both species have an entirely black crown with a wispy nuchal crest. And when in molt the Royal could show a pattern resembling the Elegant's. This problem is relieved somewhat by a difference in the species' schedule of molt: the Royal acquires its black cap in February, then loses it in May and early June. By late June all the birds have gone "bald." Elegant Terns have their black caps when they return in March and don't begin losing them until late June; some still have black mottling on the crown in mid August. By September they have finished molting.

Finally, the Elegant Tern normally gives its identity away by call long before you have a chance to study the details of its plumage or structure. It is very noisy, calling "karreck, karreck, karreck...." When the birds are in a flock, and all calling simultaneously, as happens so often, the resulting din reveals them as Elegant Terns at a distance of a quarter mile. This call also exposes the very close relationship of the Elegant with the Sandwich Tern, which has an identical call. Indeed, the French name of the Sandwich Tern, Sterne caugek, is based on it (caugek is pronounced ko-ZHEK). The only records of the Sandwich Tern in San Diego County, in 1980, 1982, 1985, and 1987, were of an individual that courted Elegant Terns in the colony at the works, and in 1995 another hybridized with an Elegant in the colony at Bolsa Chica. The Royal Tern, by contrast, is a notably silent species--if you hear large orange-billed terns making a lot of noise, they are undoubtedly Elegant.


Elegant Tern
Black: Nesting confirmation
Gray: Other records during breeding season



Royal Tern
Black: Nesting confirmation
Gray: Other records during breeding season

The two species differ radically in their seasonal status in southern California. The Elegant Tern follows a migration schedule sharply defined by the calendar: arrival in early March, a postbreeding influx in June, July, and August, and departure in October and November. Huge flocks accumulate by midsummer at the coastal lagoons. The highest estimate in our atlas database, of 3500 at San Elijo Lagoon on 8 August 1998 by Barbara Moore, is actually pretty representative. Seldom do any remain by the middle of December. In the winter of 1999-2000, Robert Patton noted two individuals remaining in north San Diego Bay (S8) as late as 5 January, one until 20 January, providing one of the very few records of the species anywhere in California in January or February.

The Royal Tern, by contrast, does not have a strong seasonal pattern, being found along San Diego County's coast year round and being at least as numerous in winter as at other seasons. The largest counts in our atlas databases are in winter: 56 at Los Peñasquitos Lagoon (N7) on 26 December 1999 by Barbara Moore, 40 at La Jolla (P7) on 28 January 2000 by Leslie Polinsky, and 60 on the Silver Strand (T9) on 19 December 1998 by Oz Osborn. Both species are confined strictly to salt water, wandering over the ocean to feed but virtually never reaching fresh water inland as the Forster's and Caspian Terns do so commonly. The only violations of this rule I know in San Diego County are a couple of records of the Elegant along the San Diego River in Mission Valley, one supported by a bird picked up on 19 October 1990 and now a specimen in the museum's collection.

The Royal Tern is a widespread species around the world, along tropical and subtropical coasts. The Elegant Tern, on the other hand, has a remarkably restricted distribution. Only five recently active colonies are known: Isla Rasa in the Gulf of California (with around 22,500 pairs, 90% or more of the known population), Isla Montague in the Colorado delta, San Diego Bay, Bolsa Chica in Orange County, and Los Angeles harbor. The last is a new colony founded in 1998 probably by birds relocating from San Diego Bay and/or Bolsa Chica.

The San Diego Bay colony was first discovered in 1959 and grew gradually from 31 pairs that year to 1,870 by 1995. In 1997 and 1998, however, hardly any Elegant Terns nested there, only about 7 in 1997 and fewer than 15 in 1998--perhaps as a result of El Niño, perhaps as a result of predation and disturbance. In 1999, with the onset of La Niña--and improved predator-management effort--the colony was back up to more usual numbers, with a maximum of 3100 nests on 27 May. Thus we see a striking illustration of the climate cycle favoring marine and terrestrial birds in alternation.

By 3 May 2000 there were 81 active nests with eggs, but some had suffered predation by Western Gulls and possibly coyotes, shifting the colony to the west.

In 1998 Brian Foster and Robert Patton discovered an unsuccessful attempt of two pairs of the Elegant Tern (along with six of the Caspian) on the jetty at Zuñiga Point at the mouth of San Diego Bay, the first known attempt at that site. All eggs except one of the Caspian suffered predation, and on 5 June Robert collected the two broken Elegant eggs for the museum. He reports that both Elegants and Caspians are making scrapes here again this year.

The Royal Tern has never nested numerously at the salt works. The first report was of one pair in 1959, and only a few were known through the 1980s. In 1997 only about four pairs laid eggs. In 1999, Robert Patton noted 35 nests on 27 May and eight nests with eggs plus eight chicks on 23 June, possibly the largest number of nesting Royal Terns yet recorded at San Diego.

The Elegant Tern's regular migration schedule contrasts sharply with its ability to shift colony sites, presumably as the fish on which it feeds shift. The dramatic but still poorly understood changes in the status of both the Elegant and Royal Terns through the 20th century reflect changes in the numbers and distribution of their prey, sardines and anchovies. The changes in these fish, in turn, follow oceanographic and climate changes that are only now beginning to be understood. The distribution and nesting success of seabirds reflect changes in the environment quickly and visibly, and the Elegant and Royal Terns are in the front line of the seabirds responding to those changes.

Thanks to Brian Foster, Elizabeth Copper, Mónica Alfaro, Brian Collins, and Robert Patton for recording much of this information, since 1999 under contract with the U.S. Fish and Wildlife Service, and to Robert for reporting it and reviewing this article.

--Philip Unitt, from the summer 2000 issue of WRENDERINGS


Focus On ... | Summer 2000 Wrenderings | Bird Atlas Introduction