Swallow-tailed gull and hunting over the open oceans, and migrating eastward to the coasts of Ecuador and Peru. The swallow-tailed gull is unique among the gulls in feeding exclusively nocturnally, mostly on fish and squid which rise to the surface at night to feed on plankton. It leaves the colony as a flock at dusk, with a great deal of screaming and display. Calls and displays are quite different from other gulls, most resembling the vocalisations of the black-legged kittiwake and Sabine's gull. The loudest and most commonly heard call is an alarm referred to as "rattle-and-whistle", a gurgling scream made with
Swallow-tailed gull photoreceptor cells. 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' nocturnal activity. The swallow-tailed gull is a near-endemic breeding bird of the Galápagos Islands, although a few pairs nest on Malpelo Island off the coast of Colombia. When not breeding, it is totally pelagic, flying