It is the only species in the genus Creagrus, which derives from the Latin Creagra and the Greek kreourgos which means butcher, also from kreas, meat; according to Jobling it would mean "hook for meat" referring to the hooked bill of this species.

[4] It is the only fully nocturnal gull and seabird in the world,[5] preying on squid and small fish which rise to the surface at night to feed on plankton.

In the breeding season, the adult has a black plumaged head and a bright red fleshy rim around each eye.

It is still unknown whether the melatonin levels are a cause or an effect of the swallow-tailed gulls' nocturnal activity.

When not breeding, it is totally pelagic, flying and hunting over the open oceans, and migrating eastward to the coasts of Ecuador and Peru.

[5] The nest is made on a small platform on a cliff, usually less than 10 m above sea level,[4] by covering the rocky ground with small pieces of lava, white coral, and sea urchin spines, which prevent the egg from rolling.

[5][9] They are asynchronous breeders (can breed any time of the year), and follow a nine-month cycle, or less if an egg or chick is unsuccessful.