Elegant Tern (Sterna elegans)

Elegant Tern crèche By J. Konecny

            The Elegant Tern is one of the group of crested terns and has the longest crest of all. Their slender yellow-orange bill appears to droop slightly at the tip. The adult in summer plumage has a black cap with long nuchal crest. The under parts may have a pink tinge, which in such cases is diagnostic in comparison with other more red- billed Terns. Some have darker central primaries contrasting with paler surrounding ones. The adult in winter (basic) plumage has a white head merging diffusely with black mask from the eye to the nape. Elegant Terns possess a black loral triangle in front of the eye, which passes directly into the post-ocular mask. Thus the eye is not isolated by white. Only at a close range is a narrow white eye ring visible.

            The Elegant Tern has the most restricted breeding distribution of any tern in North America. It is currently found in only five colonies in North America: Isla Rasa and Isla Montague, in the Gulf of California, Mexico, and the San Diego saltworks, Bolsa Chica, and Pier 400 Terminal Island, in southern California.

              Departure from southern California nesting colonies begins in late August or early September. This is often preceded by an influx of birds from other colonies, presumably in Mexico. Many range north in late summer and autumn, commonly to San Francisco and Humboldt Bay. They are much less common further north, but may reach Oregon and western Washington during El Niño years. Although some linger until November/December, most depart south in early fall to winter in coastal Ecuador, Peru, and Chile where a few remain throughout the year. Elegant Terns occur as vagrants to east Texas and the Atlantic coast. Restricted to littoral marine habitat, Elegant Tern migrations are restricted to coastal marine waters.

            In southern California, Elegant Tern foraging occurs primarily >8 km from the colony. They can be found foraging in most bays and protected areas of north San Diego County: including, but not limited to, the mouth of the Santa Margarita River, Oceanside Harbor, Buena Vista Lagoon, Agua Hedionada Lagoon and San Elijo Lagoon, south to La Jolla Cove and Mission Bay. They are typically found foraging singly or in groups of two or three. Elegant Terns are, at times, also found foraging in the outer evaporating ponds of the San Diego Bay saltworks. The diet of the Elegant Tern consists entirely of fish, the bulk of which are caught in marine waters. Those nesting at the San Diego saltworks consumed northern anchovy, topsmelt, bay anchovy, jacksmelt, jack mackerel, and long-jawed mudsuckers. Northern anchovies made up 86.8 % of their diet in 1981 and 35% in 1995. At Bolsa Chica from 1993 to 2001, and Terminal Island from 1998, 2000 and 2002, 56.4% of identified prey were northern anchovy and 13.1% were pacific sardines. Pipefish of 2-3 species made up an additional 13.5% and topsmelt, California grunion, Pacific saury and jack mackerel were also taken in smaller amounts (< 7%). Year to year variation was noted particularly the ratio of northern anchovy to pacific sardine with higher numbers of anchovy being taken in colder water years.

            Elegant Terns court and form pairs while still on migration and away from the nesting colony. In general, courtship and copulation do not occur at the nesting colony nor do the birds assemble in large numbers more than a few days prior to commencement of egg laying. They arrive at the nesting grounds and then continue courtship and pair formation in small groups or flocks called “clubs” close to but not at the nesting colony. Elegant Terns are highly colonial nesters; large groups of pairs dig shallow nest scrapes and lay eggs in one 24-hour period, with about 10 nests per square meter. At this density, nests are only 30-32 cm apart (measured from center to center) in a compact hexagonal pattern. At the San Diego saltworks, Elegant Tern egg laying commences on or about 19 April and extends through 10 June with the peak of nest initiations occurring during the week of 10 May. Nest initiation dates at Bolsa Chica and Terminal Island are about one to two weeks later than this. Elegant Terns prefer small, isolated, and undisturbed islands in Mexico. They nest in the presence of other breeding gull or tern species. Nests may have small twigs, bones, clam and snail shells that are within reach of a sitting tern pulled in to partially line the nest or they may simply be unlined shallow scrapes in the ground where they most often lay a single egg; two egg clutches are regular but it is not always sure that these are not the result of two females laying in the same scrape. During incubation and chick brooding periods the terns defecate while on the nest, and by the end of the season the substrate of each nesting group has a honeycombed white “pavement” of fecal material. Some of the diverse nesting habitats reported include; sandy and marshy coastal islands, sandy islets in salt lakes. At Bolsa Chica and Terminal Island they nest on the bare sandy substrate. Incubation times range between 23 and 27 days. Most chicks remain in the nest for about four days before becoming ambulatory and joining other chicks to form a dense moving creche. Young exhibit an extraordinarily long period of dependence on their parents, extending well over six months post fledging.

            Although Elegant Terns breed at five different sites in North American, three of which are found in southern California, possibly 90 to 97% of the world population of this tern breeds on Isla Raza, Mexico.

            The San Diego population has increased from approximately 30 pairs in 1959 to over 800 pairs in 1981. No nesting occurred at the Salt Works in 1990, but breeding was reestablished in 1991, with 250 pairs of Elegant Terns producing approximately 200 chicks. Nesting decreased at the Salt Works between 1993 and 1997 but between 312 and 427 pairs of Elegant Tern nested there in 1993, and only80 pairs were estimated to be present in 1994 and there were only two nesting attempts in 1997. The nesting population again surged in 2003 with perhaps as many as 10,000 pairs nesting there.

            In Orange County, Elegant Terns were common post-breeding migrants until 1983, when birds began to remain at Bolsa Chica throughout the spring and early summer. Their numbers increased until 1987 when 31 pairs nested (largely unsuccessfully). Bolsa Chica is now an established breeding colony and contained up to 4,000 pairs in 1995 and 1996. Smaller numbers have nested there in recent years, mostly due to the growth of the Terminal Island colony and/or disturbance at Bolsa Chica.

            In 1998, a large cohort of Elegant Terns bred at Terminal Island in the Los Angeles Harbor. The nesting area is a portion of a 15-acre nesting site provided for Least Terns and enclosed with three-foot high chick fencing. Many of these terns nesting at Terminal Island in 1998 were originally from the Bolsa Chica colony and were expected to move back there when the site was developed by the Port of Los Angeles. However, from 2000 to 2002, they successfully nested principally in two locations within the designated nesting site for Least Terns and may continue to nest here despite adjacent areas at Terminal Island having been developed for port facilities. By 2003 the total southern California population of Elegant Terns had increased to 10,000-12,000 of which 10,000 returned to breed in the San Diego saltworks largely abandoning both Bolsa Chica and Terminal Island where the bulk of the population had bred in recent years. This sudden shift was reversed in 2004 when large numbers (>10,000 pairs) of Elegant Terns nested at Terminal Island and another 1300 at Bolsa Chica. Local water conditions may have influenced this shift in colony location. Birds banded t Bolsa Chica and Terminal Island have been recovered in the other nesting colonies further substantiating a high degree of inter-colony movement from year to year within southern California. To date, no banded bird from one of the Mexican breeding colonies has been recovered in any of the southern California colonies. More distant recoveries of Elegant Terns banded at Bolsa Chica include individuals overwintering or in migration in El Salvador (2) Costa Rica (1) and Colombia (1).

            Elegant Terns are found throughout the eastern Pacific coastal region, a region that is the epicenter of El Niño events. As such, they are subject to large-scale climatic changes that impact local food resources as seen in a variety of other seabird species including locally nesting Least Terns. Growth studies performed before, during, and after the El Niño event of 1998 showed no significant impact on Elegant Tern chick growth suggesting that this species is adapted to periodic climatic shifts and short-term changes in availability of their principal prey. They may only be affected by the most severe of these climatic events.

            Even so, their limited breeding range and small number of breeding colonies is cause for some caution and has resulted in the Elegant Tern being listed as a Bird Species of Special Concern in California.

By Dr. Charles Collins for USFWS