Swallow-tailed Gull
Encyclopedia
The Swallow-tailed Gull (Creagrus furcatus) is an equatorial seabird
in the gull
family
Laridae. 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. It was first described by French naturalist and surgeon Adolphe-Simon Neboux
in 1846. Its scientific name is originally derived from the Greek word for gull, "Glaros" and via Latin
Larus, "gull" and furca "two-tined fork". It spends most of its life flying and hunting over the open ocean. The main breeding location is on the rocky shores and cliffs of Hood, Tower and Wolf Island, with lower numbers on most of the other islands. It is more common on the eastern islands where the water is warmer.
It is the only fully nocturnal
gull and seabird in the world, preying on squid
and small fish which rise to the surface at night to feed on plankton
.
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.
, 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
.
, 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.
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.
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Population trends have not been estimated, but it is not thought to be threatened. The population was estimated to be about 35,000 individuals when it was last considered in 2004.
in the back of the eye that reflects light back through the retina
, increasing the amount of light available to the photoreceptor cells.
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.
Seabird
Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...
in the gull
Gull
Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...
family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Laridae. It is the only species
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
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
Beak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
of this species. It was first described by French naturalist and surgeon Adolphe-Simon Neboux
Adolphe-Simon Neboux
Adolphe-Simon Neboux, born 1806 and died 1844, was a French surgeon and naturalist whoaccompanied the frigate Vénus under command of Admiral Abel du Petit-Thouars between the years 1836 and 1839, visiting the Pacific coastline of North America and the Galápagos Islands.Among species he described...
in 1846. Its scientific name is originally derived from the Greek word for gull, "Glaros" and via Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
Larus, "gull" and furca "two-tined fork". It spends most of its life flying and hunting over the open ocean. The main breeding location is on the rocky shores and cliffs of Hood, Tower and Wolf Island, with lower numbers on most of the other islands. It is more common on the eastern islands where the water is warmer.
It is the only fully nocturnal
Nocturnal animal
Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal"....
gull and seabird in the world, preying on squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
and small fish which rise to the surface at night to feed on plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
.
Description
The Swallow-tailed Gull has no structural or plumagePlumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...
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.
Distribution and habitat
The Swallow-tailed Gull is a near-endemic breeding bird of the Galápagos IslandsGalápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
, although a few pairs nest on Malpelo Island
Malpelo Island
Malpelo Island is an island located from Colombia's Pacific coast, and approximately from Panama's coast. It has a land area of . It is uninhabited except for a small military post manned by the Colombian Army, which was established in 1986. Visitors need a written permit from the Colombian...
off the coast of Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
. When not breeding, it is totally pelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
, flying and hunting over the open oceans, and migrating
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
eastward to the coasts of Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
and Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
.
Food and feeding
The Swallow-tailed Gull is unique among the gulls in feeding exclusively nocturnallyNocturnal animal
Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal"....
, mostly on fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
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
Calls and displays are quite different from other gulls, most resembling the vocalisations of the Black-legged KittiwakeBlack-legged Kittiwake
The Black-legged Kittiwake is a seabird species in the gull family Laridae.This species was first described by Linnaeus in his Systema naturae in 1758 as Larus tridactylus....
and Sabine's Gull
Sabine's Gull
The Sabine's Gull is a small gull. Its generic placement is disputed; some authors treat it as the sole species in the genus Xema as Xema sabini, while others retain it in the genus Larus as Larus sabini. It breeds in the arctic and has a circumpolar distribution through northernmost North America...
. 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.
Breeding
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.Status
Night vision
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 lucidumTapetum lucidum
The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....
in the back of the eye that reflects light back through the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, increasing the amount of light available to the photoreceptor cells.
Melatonin
A study of melatoninMelatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
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.
Further reading
- Snow, D.W. & Snow, B.K. (1967). "The breeding cycle of the Swallow-tailed Gull (Creagrus furcatus)." Ibis 109(1):14-24
- Snow, B.K. & Snow, D.W. (1968). "Behavior of the Swallow-tailed Gull of the Galapagos." Condor 70(3):