Inca tern


The Inca tern is a near-threatened species of tern in the subfamily Sterninae of the family Laridae. It is found along the Pacific coasts of Chile, Ecuador and Perú, and has appeared as a vagrant in Central America and Hawaii.

Taxonomy and systematics

The Inca tern is the sole member of its monotypic genus Larosterna, and has no accepted subspecies. Perhaps surprisingly given its highly distinct plumage, the Inca tern is not particularly basal among the terns, being more closely related to typical Sterna terns than any of the superficially much more similar genera Gelochelidon, Hydroprogne, Onychoprion, Phaetusa, or Sternula are.

Description

The Inca tern is roughly long and weighs between. Its plumage is uniquely colored among terns; adults have a mostly dark, slate-gray body, with paler underwing coverts, and a slightly paler throat. A white stripe extends back from the base of the bill and fans out like a Salvador Dalíesque mustache as long, satiny feathers along the side of the neck. The trailing edge of the wing are white. The tail is black, and moderately forked. The iris is brown, with legs and feet that are dark red. The bill is bright to dark red, with bare yellow skin at the base. The chicks, on hatching, are purplish-brown, progressing through brownish-gray before developing mature plumage. The chicks' bills and legs are dark and horn-colored, and gradually attain the red that adults have.

Distribution and habitat

The Inca tern is an inhabitant of the Humboldt Current region. It breeds from Lobos [de Tierra], in northern Perú, south to the Aconcagua River, near Valparaíso, Chile. Some disperse north into Ecuador after breeding, and south to the Puerto Montt area of Chile, where multiple 'research-grade' sightings with photos on the biodiversity database iNaturalist, including a group of four together. It is a casual visitor to Panamá and Costa Rica, and has also been recorded as a vagrant in Guatemala and Hawaii. The documented Hawaiian birds, in particular, remained from March through November on the Hawaiian archipelago.
The South American Classification Committee of the American Ornithological Society treats the Inca tern's presence in Colombia as "hypothetical", due to a number of unverified or undocumented sightings. On iNaturalist, one 'research-grade' sighting has been documented from the coast of Buenaventura, Colombia's Isla Cascajal, dated July 2023; however, the species' popularity in captivity complicates assessment of extralimital records.
The Inca tern nests on sea cliffs and guano islands, as well as manmade structures and abandoned barges. It will gather with other sea and shorebirds on sandy beaches.

Behavior

Movement

The Inca tern is essentially non-migratory, although some disperse northwards after breeding. Some rogue individuals have flown great distances.

Feeding

The Inca tern feeds primarily on small fish, such as anchoveta, and also consumes planktonic crustaceans and offal or carrion in its diet. Large flocks congregate around fishing boats and will also follow the feeding patterns of cormorants, sea lions, and whales and dolphins. Feeding flocks can number over 5,000 birds. The Inca tern catches its prey mainly by plunge-diving, but also picks items from the surface while flying or floating on the water.

Breeding

Breeding does not appear to be concentrated in any season. Eggs have been found between April and July and between October and December, and other evidence of breeding has been noted in August. It nests in a variety of sites including fissures and caves in rock cliffs, among rocks and boulders on island slopes, in abandoned petrel and penguin burrows, and on and under human structures. The clutch size is usually two eggs though sometimes one. Both sexes incubate the clutch and provision the young. The incubation period is not known; fledging occurs about four weeks after hatch and the young are fully dependent on the adults for at least a month after fledging.

Vocalization

The Inca tern is most vocal at its nesting colonies. Its calls include "raucous cackling notes" and "mewing"; the latter call has been likened to that of a kitten.

Status

The IUCN has assessed the Inca tern as Near Threatened. It has a somewhat restricted range; its population size is not known and is believed to be decreasing. "Reproductive success is dramatically reduced during El Niño events". Human harvesting of its primary prey is a probable threat as is climate change. One estimate placed its population at about 150,000 in 2011.

Captivity

Because of its unusual plumage, the species is popular in zoos. Escapes from captivity can occur, which leads to problems determining genuine vagrancy.