Elegant tern

Thalasseus elegans is a tern of the family Laridae, described by Gambel in 1849 under the name Sterna elegans ^[1]. In Peru it is called «gaviotín elegante» ^[2]. Its breeding range is very small: islands in the northern Gulf of California and a few colonies on the shores of southern California. Outside the breeding season it spreads along the Pacific shores between Nayarit, in Mexico, and southern Chile ^[3]. The IUCN Red List classifies it as Near Threatened in its 2020 assessment ^[4].

It is closely related to Cabot's Tern (Thalasseus acuflavidus): some authors treat them as sister species, others place both in one superspecies; in the Isla Rasa colonies, birds with features intermediate between the two have been seen, with an orange bill smudged with black and a paler, orange-yellow tip ^[5]. Compared with Cabot's Tern, the Elegant Tern is larger, has a longer crest and a long, thick bill with no black at all ^[5].

Similar species

• Thalasseus sandvicensis: Cabot's Tern, which some authors treat as a subspecies of Thalasseus sandvicensis, is smaller, with a short crest, black legs and a short, slender bill that is black except for the tip; the Elegant Tern's bill is long, thick and without black ^[5].

For decades Isla Rasa, in the Midriff Islands region of the Gulf of California, was regarded as holding more than 95 % of the species' world breeding population ^[6]. From there, breeding has spread northward: in May 1959, 31 pairs were found nesting in the saltworks of San Diego Bay, a colony formed at Bolsa Chica in 1987 and another in Los Angeles Harbor in 1998, and by 2014 the region held more than 50000 nests ^[6].

After breeding, some birds move up the coast as far as British Columbia, while most spend the northern winter between Ecuador and northern Chile ^[3]. It also turns up far from that route: it was recorded on Johnston Atoll, in the central Pacific, in 1969, and in France in 1974, and since 2009 a few pairs have bred in western Europe ^[3].

In the bird inventory of the San Andrés beaches, in Pisco, it is listed as a Nearctic migrant and was recorded on the stony and rocky shores ^[7]. On the sandy beach of Pozo Lizas, in Ilo, it was seen in March 2009 alongside gulls of several species ^[8].

On Isla Rasa it nests every year in mixed colonies with the Royal Tern (Thalasseus maximus); the Elegant Tern colony numbers around 100000 nests and the population breeding on the island approaches 200000 individuals ^[5]. On the flat ground where it settles, the mean density is 15 nests per square metre, and because the birds stand up to defecate, a rim of guano builds up around each nest and outlines the colony ^[6].

Before 2000, breeding failures on Isla Rasa, when birds did not lay, abandoned the colony or lost many chicks, coincided with large-scale El Niño events such as those of 1992 and 1998 ^[6]. Since then they respond to local warming: in years when the sea temperature anomaly in the Midriff exceeded 1 °C (2003, 2009, 2010 and 2014), more than 70 % of pairs nested in the California colonies, whereas in other years that share never went above 20 % ^[6].

In 2010 and 2011, 15 birds that appeared to be hybrids with Cabot's Tern were identified on Isla Rasa, including 3 chicks ^[5]. Of 1779 adults counted in the observation areas, 10 were presumed hybrids, about 0.56 %, and some of them were raising chicks, which indicates that these hybrids are fertile ^[5].

It feeds on small pelagic fish ^[6]. In regurgitations collected on Isla Rasa over 23 seasons, Pacific Sardine (Sardinops sagax) and Northern Anchovy (Engraulis mordax) together made up on average 92 % of its diet, with Pacific mackerel the third most eaten prey ^[9]. A stable isotope analysis of chicks raised between 2017 and 2019 showed that adults brought them mainly Northern Anchovy ^[10].

Because the tern eats young sardines that the fleet will catch months later, the share of sardine in its spring diet is related to the catch per unit effort of the following fishing season; after the peaks in sardine consumption in the springs of 1988, 1996, 2001 and 2008, that share dropped, and the fleet's catch per unit effort fell one or two seasons later ^[9]. The first major collapse of the Midriff sardine fishery, between 1991 and 1993, coincided with less sardine in its diet and with California nests rising from 250 to 5870 in 3 years ^[6].

Its global category is Near Threatened ^[4]. Because such a large part of the population gathers on a single island, rats and mice preying on eggs and chicks on Isla Rasa came to threaten the species with extinction ^[3].

Black rats (Rattus rattus) and house mice (Mus musculus) probably reached Isla Rasa with the guano mining of the late nineteenth century; their eradication was completed in early 1995, after which new colonies appeared on neighbouring Isla Cardonosa, with about 1000 nests in 2004 and about 15000 in 2008 ^[6]. After that eradication, the population breeding on Isla Rasa and in southern California grew almost immediately ^[3].

Sardine fishing also weighs on its choices: the share of birds leaving the Midriff to breed elsewhere rises with the sardine fishing effort and landings of the previous season, and the protection of the California colonies has probably helped them grow ^[6].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In fresh droppings of this species and of Heermann's Gull (Larus heermanni) collected on Isla Rasa, researchers isolated Escherichia coli, with no sign that the strains were pathogenic, and bacteria related to Vibrio parahaemolyticus, Vibrio alginolyticus and Staphylococcus aureus, species that can make people ill ^[11]. According to the authors, none of those isolates belonged to the species or strains of greatest importance for human health ^[11].

When guano mining on Isla Rasa ended, early in the twentieth century, commercial collecting of seabird eggs on the island began; at its peak, local fishers took some 500000 eggs, and the practice, banned in the 1960s, ended in the early 1980s ^[6].

Its diet works as an early warning for the fishery: together with the Southern Oscillation Index, the share of sardine in its spring diet made it possible to predict the sardine catch per unit effort of the Gulf of California fleet in the following season ^[9].

In the north, the beaches of the Illescas National Reserve are a seasonal stopover for at least 23 species of migratory birds, the Elegant Tern among them, according to its 2024 master plan ^[2]. At the Poza de La Arenilla coastal wetland, in La Punta (Callao), it was recorded in all 4 seasons of the year in counts made from 2013 to 2018 ^[12].

Further south, in the bird censuses of the San Andrés beaches, in Pisco, next to the northern end of the Paracas National Reserve, in its buffer zone, a total of 76 Elegant Terns were counted in the January 2023 census and 359 in the December 2024 census ^[7]. On Pozo Lizas beach, in Ilo, near Punta Coles, it was observed in March 2009 ^[8].