Swallow-tailed Gull

How to find it

Plan around a sea crossing to Isla del Coco, the main chance of meeting this gull. Think of the dark hours rather than daytime. Birders working on a boat near Coco saw it following the vessel at night and before dawn. Watch the moments when the boat enters or leaves the island, as that is when birds trailed behind. Keep expectations modest, because it lives over open ocean far from land.

How to recognise it

Start with the tail. It is deeply forked, and no other gull has this.

In flight the upperwing shows a triangle pattern: grey back, white inner wing and black outer wing.

The head depends on the bird's breeding cycle. Breeding adults have a dark grey head with a red ring around a large eye. Non-breeding adults have a white head with a dark patch around the eye. In both plumages the bill is black with a pale tip.

Birds in Costa Rican waters are away from their colonies, so the white-headed, non-breeding look is the one to expect. These marks describe the species in general.

Behaviour and diet

This is the only gull that feeds entirely at night. It takes squid and small fish that rise to the surface after dark, and it catches them by plunging at the surface.

Around Isla del Coco, researchers saw it on every expedition. Birds followed the research vessel when it entered or left the island, during the night and the early hours before dawn. That habit fits a gull that hunts only after dark.

Where it lives

Away from its breeding colonies it lives fully at sea over open ocean. It travels along the Pacific coast of South America, from central Colombia to central Chile.

In Costa Rica it is reported often near Isla del Coco National Park and in the waters around it, far offshore. Costa Rican waters lie at the northern edge of its non-breeding dispersal.

When to see it

A survey of seabirds off the Pacific coast of Costa Rica says some Galapagos gulls regularly disperse to Costa Rican waters from late March to early May. Galapagos birds can breed in any month, so their timing at the colonies is not seasonal.

Similar birds

The deeply forked tail sets it apart, as no other gull has this. The triangle pattern on the upperwing in flight, with grey back, white inner wing and black outer wing, adds a second clue.

Names and lore in Costa Rica

Gaviota Tijereta is the name on the Costa Rican list. Other Spanish names for the species are gaviota de las Galápagos, gaviota rabihorcada and gaviota de cola bifurcada. A Costa Rican seabird article calls it the only nocturnal gull in the world.

Status and conservation

The Costa Rica list marks it M, a migrant, though its visits are movements outside the breeding season to find food, and the IUCN lists the species as Least Concern.

Photography tips

Dark-hours boat travel is the likely time to see it, not daytime. Birders working on a boat near Coco saw it following the vessel at night and before dawn, so expect low light and brief views.