Inca Tern

The Inca tern is a tern found along the Pacific coasts of Chile, Ecuador and Peru. It is an inhabitant of the Humboldt Current region and breeds from northern Peru south to near Valparaíso, Chile. It is 39 to 42 cm long and weighs between 180 and 210 g. Adults have a slate-grey body, a bright red bill with bare yellow skin at the base, dark red legs and white wing edges. It feeds mainly on small fish such as anchoveta, along with planktonic crustaceans and offal, catching prey mostly by plunge-diving. Large flocks gather around fishing boats and follow cormorants, sea lions, whales and dolphins. It nests in rock fissures, caves, and abandoned petrel and penguin burrows.

About the Inca tern

The Inca tern is a medium-sized seabird of the gull and tern family, Laridae, found along the Pacific coasts of Peru and Chile. It is the only species in its genus and is unlike any other tern, with dark slate-grey plumage, a red bill and a white "mustache" of long feathers fanning out on each side of the face.

Its scientific name is Larosterna inca, and it has no recognized subspecies. Despite its unusual look, it is not an early offshoot of the tern family. It is actually more closely related to the typical Sterna terns than several other tern genera that look much more like them. Terns as a group belong to the subfamily Sterninae and are close relatives of gulls and skimmers.

The bird measures 39–42 cm (15–17 in) long and weighs 180–210 g (6.3–7.4 oz). That puts it in the middle of the tern size range, which runs from the least tern at 23 cm (9 in) to the Caspian tern at 48–56 cm (19–22 in).

Most sea terns are pale grey or white with a black cap. The adult Inca tern is the opposite: its body is mostly dark slate-grey, with a slightly paler throat and paler feathers under the wings. The trailing edge of the wing is white, formed by the tips of the secondary feathers and the three inner primaries. The tail is black and moderately forked. The eye is brown, and the legs and feet are dark red.

The face is the showpiece. A white stripe runs back from the base of the bill, then fans out into long, satiny feathers along the side of the neck, a flourish that has been compared to a Salvador Dalí mustache. The bill is bright to dark red, with bare yellow skin at its base. Chicks hatch purplish-brown, turn brownish-grey, and only gradually gain the adult plumage. Their bills and legs start out dark and horn-colored and turn red over time.

Behavior and diet

The Inca tern feeds mainly on small fish, especially anchoveta, and also takes planktonic crustaceans, offal and carrion. It hunts mostly by plunge-diving from the air into the sea. It also snatches food from the surface while flying, or while floating on the water. Like other terns, it cannot focus its eyes underwater, so it relies on spotting prey accurately from above before it dives.

These terns are highly social feeders. Huge flocks, sometimes more than 5,000 birds, gather around fishing boats. They also follow the feeding patterns of other hunters, including cormorants, sea lions, whales and dolphins. On land, they mix with other seabirds and shorebirds on sandy beaches.

Inca terns are noisiest at their nesting colonies. Their calls include "raucous cackling notes" and a mewing call that has been likened to a kitten.

Breeding does not seem to be tied to any one season. Eggs have been found from April to July and from October to December, with other signs of breeding in August. Nest sites vary widely. Inca terns nest in cracks and caves in rocky cliffs, among boulders on island slopes, in abandoned petrel and penguin burrows, and on or under human structures such as ledges beneath piers and old barges.

The clutch is usually two eggs, sometimes one. Both parents incubate and both feed the young. The incubation period is not known. Chicks fledge about four weeks after hatching but depend completely on their parents for at least another month.

Range and habitat

The Inca tern belongs to the Humboldt Current region, the cold, nutrient-rich ocean current that stretches from southern Chile to northern Peru. It breeds from Lobos de Tierra in northern Peru south to the Aconcagua River near Valparaíso, Chile. After breeding some birds move north into Ecuador or south to the Puerto Montt area of Chile. The species is essentially non-migratory, though.

Strays turn up farther afield. The Inca tern is a casual visitor to Panama and Costa Rica and has been recorded as a vagrant in Guatemala and Hawaii, where documented birds stayed from March through November. Its presence in Colombia is treated as hypothetical, and because the species is often kept in captivity, escaped birds complicate the picture for sightings outside the normal range.

The Humboldt Current explains why the tern lives where it does. Cold water welling up from the depths makes this the most productive current system of its kind, and it accounts for roughly 18–20% of the world's marine fish catch. Sardines and anchovies swarm in these waters, supporting eared seals, whales and dolphins, and seabirds. Off Peru, surface water can be as cold as 16°C (61°F), unusually cool for the tropics.

Conservation status

The Inca tern is listed as Near Threatened on the IUCN Red List, in a 2018 assessment by BirdLife International. Its range is fairly restricted, its total population size is not well known and it is believed to be decreasing. One estimate put the population at about 150,000 in 2011.

El Niño is a major natural problem for the tern. During El Niño events, the productivity of the Humboldt Current drops and fish stocks can crash, and the tern's breeding success falls dramatically. In 1972, for example, an El Niño event caused Peru's anchoveta population to collapse after a fishery boom that had topped 12 million tons a year.

People add to the pressure. Commercial fishing of anchoveta, the tern's main prey, is a probable threat, and climate change is another. Across the tern family, most species are declining because of habitat loss, pollution, disturbance and introduced predators, all linked to human activity.