Forster's Tern

How to find it

Start with the places where it has been reported: the Gulf of Nicoya near Chomes, and the Tempisque basin at Puerto Humo and Palo Verde. Palo Verde is reached on guided boat trips along the Tempisque River, with a naturalist guide aboard. The dry season brings hundreds of migratory bird species there, which makes it the likeliest setting for a winter visitor like this tern. Look for single birds or small groups over water, and check any pale tern for the isolated black eye patch. Treat it as a bonus rather than a target.

How to recognise it

Forster's Tern is a medium-sized tern, about 37 cm long, with a deeply forked tail.

In winter the adult shows:

- a white crown

- a wide black band running from the eye to the ear, its most distinctive mark

- white on the rest of the head and the underparts

- a pale gray back

- a dull bill, brownish with a black tip

- orange-brown legs

A breeding adult looks quite different, with a black cap, an orange bill and reddish legs. Since Costa Rican reports are from winter, the eye patch is the mark to expect.

Young birds look like winter adults but have a scaly, gingery-brown pattern on the back and wing coverts.

What it sounds like

Recording: fake_id, CC0 1.0, source, read 4 Nov 2026.

Behaviour and diet

In Costa Rica Forster's Tern is usually found alone or in small groups, so do not expect a noisy flock.

Across its range it catches small fish by plunge-diving or snatching them from the surface. The fish are usually small, about 2.5 to 10 cm long, and make up most of the diet in every season. It also hawks insects in flight, so a bird working over a lagoon may be watching for more than fish.

Where it lives

In Costa Rica it visits saltwater ponds, freshwater marshes, lakes, lagoons and rivers. It is a bird of open water and water edges rather than of forest.

Reports come from two parts of the country: around the Gulf of Nicoya, at Chomes, and in the Tempisque basin, at Puerto Humo and Palo Verde. Palo Verde National Park lies on the Tempisque River. On its wider winter grounds it stays mostly near the coast, in estuaries, inlets, coastal lagoons and sheltered bays, though some birds use inland marshes, lakes and rivers.

When to see it

Costa Rican reports of this tern fall between November and February, which makes it a winter bird here.

Similar birds

The look-alike is the Common Tern. Forster's has a thicker bill, paler primaries and a slightly longer tail that extends past the wingtips when perched. Its tail is darker in the center, not on the outer edge. In winter the Common Tern keeps a black nape and has black legs, while Forster's black eye patch is isolated, does not join across the back of the neck, and its legs stay orange. Forster's also looks paler underneath, with less dusky shading below, and is slightly bulkier and broader winged.

Names and lore in Costa Rica

In Costa Rica the name is Charrán de Forster. Other Spanish names used for the species elsewhere include Charrán Cejinegro (Nicaragua), Gaviotín de Forster (Panama), Gaviota de Forster, Gaviotín de Alas Blancas and Golondrina del Mar.

Status and conservation

The official Costa Rican bird list gives Forster's Tern the status M, a migrant, and here it is a casual or rare winter visitor.

Photography tips

A lone pale tern hovering and then dropping is a good cue to stop and check the head pattern. Aim for the winter head, with its isolated black eye patch on a white crown, which separates it from the Common Tern.