Gabbiano codadirondine

Identificazione

The swallow-tailed gull has no structural or plumage differences between the male and female. In the breeding season, the adult has a black plumaged head and a bright red fleshy rim around each eye. Outside the breeding season, the head is white and the eye rim becomes black. It has a grayish upper breast, gray mantle, and black wingtips. The mostly black bill has a contrasting white tip.<p>In order to see while hunting for food at night, the swallow-tailed gull's eyes are larger in size and volume than those of any other gull. They also possess a tapetum lucidum in the back of the eye that reflects light back through the retina, increasing the amount of light available to the photoreceptor cells.<p>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.

Tassonomia

Nome scientifico

Creagrus furcatus

Autorità

(Néboux, 1842)

Ordine

Caradriformi

Famiglia

Laridae

Genere

Creagrus

Areale e habitat

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 and hunting over the open oceans, and migrating eastward to the coasts of Ecuador and Peru.

Areale riproduttivo

SA : Galápagos and Malpelo I. (far w of Colombia)

Areale non riproduttivo

to Humboldt Current off Ecuador to n Chile

Migrazione

Complete migrants

Comportamento

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.<p>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 the head moving side to side. This call is contagious, with other birds joining in without seeing the cause. A loud and rapid kweek, kweek, kweek is the greeting call between mates, made with the head and neck curved forward to the ground.<p>The swallow-tailed gull breeds from about 5 years old, with pairs frequently staying together from year to year. Most breed throughout the year in mixed colonies on the cliffs of the Galápagos Islands sometimes on flat areas, and food for the young is hunted from the seas near to the nesting colonies. The nest is made on a small platform on a cliff, usually less than 10 m above sea level, by covering the rocky ground with small pieces of lava, white coral, and sea urchin spines, which prevent the egg from rolling. Nesting birds tend to face the cliff, a habit common among exclusively cliff nesting gulls, such as the black-legged kittiwake. The female usually lays one speckled egg per breeding attempt. 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. The egg is generally incubated for 31–34 days. A chick takes its first flight at about 60–70 days old, and is fed by the adults until about 90 days, when it leaves the land, possibly with the adults, to live over the open seas.

Conservazione

Tendenza della popolazione

unknown