The Swallow-tailed Gull (Creagrus furcatus) is an equatorial seabird in the gull family Laridae. It is the only species in the genus Creagrus. It was first described by French naturalist and surgeon Adolphe-Simon Neboux in 1846, its specific epithet derived from the Latin furca "two-tined fork". The species is almost entirely a breeding endemic of the Galapagos Islands, although a few nesting pairs breed on Malpelo Island off the coast of Colombia. When it is not breeding it is totally pelagic (flying and hunting over the open oceans), migrating eastward to the coasts of Ecuador and Peru. A notable aspect of their biology is their night foraging behaviour, feeding on squid and small fish. They are the only fully nocturnal gull and seabird in the world. Adaptions include night-vision, their eyes are larger in size and volume than any other lariid. They also have the tapetum lucidum in the back of the eye that reflects light back through the retina, increasing the amount of light available for it to capture. These seabirds breed in mixed colonies on the cliffs of the Galapagos Islands. They are asynchronous breeders (can breed any time of the year) but follow a 9 month cycle. They are sexually monomorphic, meaning there are no morphological or plumage differences between the males and females. In the breeding season both sexes have black plumage on their head and the fleshy rim around their eyes turns bright orange. Outside the breeding season their head plumage turns white and the eye rim goes black.

It is unique within the gulls for feeding exclusively at night, feeding mostly on squid, which rise to the surface at night to feed on plankton. It breeds colonially throughout the year; unlike most other gull species it lays a single egg per breeding attempt. The egg is incubated for 32 days, and brood the chick for 3 months.

A study of melatonin levels of Swallow-tailed Gulls found them to have no measurable daily melatonin rhythm, whereas a day-feeding gull that was chosen for comparison had the expected higher night time melatonin level for day active birds. High melatonin levels generally make birds sleepy. It is still unknown whether the melatonin levels are a cause or an effect of the Swallow-tailed Gulls night time activity.

On rocky ground, Galapagos Islands

Flying, Galapagos Islands

Close-up of head and neck

A Swallow-tailed Gull chick