Inca tern

Larosterna inca is a seabird of the family Laridae described by Lesson in 1827 ^[1]. In Peru it is known as «zarcillo» ^[2]. It is a dark-bodied tern with a bright red bill, white feather moustaches and yellow caruncles at the gape, living in inshore waters by rocky cliffs and guano islands from northern Peru to central Chile ^[3]. The IUCN Red List classifies it as Near Threatened ^[4], and Peru's official list of threatened wildlife of 2014 includes it as Vulnerable ^[5].

The adult has dark grey body plumage, a bright red bill, a moustache of white plumes on each side of the face and yellow caruncles at the gape ^[3]. The moustache is long and the caruncles are fleshy ^[6]. Males and females look alike: the ornaments are similar in size and colour in both sexes, except caruncle length, which is greater in males, a difference that may simply reflect their longer heads ^[6].

Young birds are recognised by brownish body feathers with pale edges, a dark bill, small dark caruncles and a short dark moustache ^[3]. In adults caruncle colour ranges from dull orange to bright yellow, and only immatures have dark ones; in a bird in its first plumage cycle followed for months, the caruncles lightened gradually to yellow and the white moustache was the last part of the adult plumage to be completed, both by September ^[3].

In Peru it has been recorded between Lobos de Tierra Island (6° 27′ S) and the port of Ilo (17° 31′ S), according to the earlier work compiled in an IMARPE study ^[7]. It is resident and makes no regular migrations, though it may move to follow food, especially during El Niño episodes, and it regularly reaches Ecuador ^[3].

Beyond that range it turns up as a vagrant: it has been seen in Panama, in Costa Rica and, on 27 April 2019, in Guatemala ^[3]. Between 10 March 2021 and 8 January 2022 a single bird spent 305 days moving among the Hawaiian Islands, the northernmost and westernmost record of the species and the first outside the Americas ^[3]. In the Galápagos Islands the first documented records are an adult on Daphne Major on 14 August 2008 and another that landed on a boat deck near Gardner Bay, Española, on 2 April 2017 ^[8].

It is monogamous, and male and female share incubation and chick feeding ^[6]. The length of the white moustache proved the best indicator of body condition in both sexes and was related to each adult's breeding outcome, from not breeding to raising 1 or 2 chicks; in addition, the mass chicks reached and their immune response were related to the moustache length of their parents, which supports the idea that both sexes use these ornaments to display their state ^[6].

Unlike most terns, which nest on open ground, the Inca tern breeds inside crevices ^[9]. In a colony under heavy predation by the peregrine falcon (Falco peregrinus), pairs chose holes with longer chambers, more cavities and more cover above and at the sides, close to the edge of the cliff; most breeding failures were due to predators, nests far from the edge were attacked more, and parents there fed their chicks less often, so the chicks were lighter ^[9]. Adults reached the colony in flocks that scattered on reaching the cliff edge, probably as a response to predators ^[9].

The breeding season is long and many pairs raise two broods a year ^[3]. In the bays of Paracas and Independencia it has been described as nesting all year, most intensely between September and November; in the San Fernando National Reserve it uses salt crusts along the cliff edges, cracks in the slopes and gaps between rocks at the cliff foot as possible nest sites ^[7].

It feeds mainly on small fish, usually anchoveta (Engraulis ringens), and also on planktonic crustaceans and on scraps and offal left by fishing, so it often approaches boats ^[3].

Observers aboard the industrial anchoveta purse-seine fleet recorded top predators in 12778 hauls between 2015 and 2019; the Inca tern was present in 26.69 % of them and was the most frequently sighted seabird in the whole study ^[2]. The authors suggest that the prey concentration created by the industrial fleet is especially attractive to species such as the Inca tern and the pelican ^[2]. On a July 2010 cruise off the northern and central coast, between 7° 00′ S and 10° 59′ S, 53.38 % of the Inca terns observed came close to the ship ^[10].

Its global category is Near Threatened ^[4]. In Peru it is classified as Vulnerable, and the regulation that lists it prohibits the hunting, capture, possession, trade, transport and export for commercial purposes of the species included ^[5].

An IMARPE study notes that, according to the IUCN, its population is declining ^[7]. In the San Fernando National Reserve feathers were found in salt crusts on the cliffs, a sign of possible nests, but in a full year of monitoring no pair with eggs or chicks was seen; the authors suggest that tourism at Punta Gallinazo and seaweed collection at Pingüinera may have led the birds to stop breeding there ^[7].

During the H5N1 avian influenza outbreak that left at least 55000 seabirds dead on Peru's beaches and islands between November 2022 and January 2023, a juvenile Inca tern found dead on Venecia beach, Villa El Salvador (Lima), in December 2022 tested positive for the virus; it was the first confirmed detection of H5N1 in this species in Peru ^[11].

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

The Inca tern is one of the hosts of the soft tick Ornithodoros amblus, collected at 13 localities on the coast and islands of Peru together with pelicans, boobies and cormorants ^[12]. This tick bites people readily and its bite causes strong inflammation and intense itching; the Punta Salinas and Huacho viruses have also been isolated from it ^[12].

In the 2022 outbreak, highly pathogenic H5N1 avian influenza virus was detected in a juvenile Inca tern found dead on a beach in Lima, and the authors of the finding call for continued surveillance under a One Health approach because of the risk to wildlife and to human health ^[11].

In an experiment with 8 mesocosms moored about 6 km off Callao between 25 February and 16 April 2017, Inca terns began to roost between the anti-bird spikes on the roofs ^[13]. From day 37 the extra bird scarers did not work, and in the last 2 weeks there were often more than 10 birds on each mesocosm; their droppings fertilised the water, relieved nitrogen shortage and set off intense phytoplankton blooms in the top 5 m, an effect the researchers called orni-eutrophication ^[13].

In monthly counts on 12 beaches of the San Fernando National Reserve, Ica, between June 2018 and May 2019, the Inca tern was the most abundant of the 29 bird species recorded ^[7]. In that reserve its possible breeding sites are on the cliffs of Punta Gallinazo, Punta Vera and Pingüinera ^[7]. Punta Gallinazo is a tourist area with viewpoints, but its cliffs are very high and access to them is restricted ^[7].

At sea it is common around fishing boats: between 2015 and 2019 it was the most frequently sighted bird during hauls of the industrial anchoveta fleet ^[2].