Swallow-tailed Gull Creagrus furcatus Scientific name definitions

The Swallow-tailed Gull is one of the most distinctive and aberrant species in its family. Though superficially similar in appearance to other species, certain aspects of its biology are quite distinctive. It breeds mainly on the Galapagos Islands and one island off Colombia and when not breeding travels along the off shore waters of South America’s Pacific coast from central Colombia to central Chile. It breeds asynchronously all year long in many different colonies located on steep slopes or broken cliffs. While breeding, both parents remain at the nest during the day, alternating brooding duties. At night the non-brooding parent will take to sea and forage the entire night for squid. This is the most nocturnal gull and its large eyes are specifically adapted for this strategy. In further contrast to almost all other gulls which will return to their natal colony to breed, at the onset of their first breeding cycle Swallow-tailed Gulls will find a new breeding colony.

51–61 cm; 610–780 g; wingspan 124–139 cm. Very distinctive .  A strikingly patterned gull with deeply forked tail ; breeding adult has dark grey head , becoming paler on upper neck and breast; white patch above base of bill; mantle, back and wings grey; rump and tail white ; underbody white; bill blackish, with pale tip; legs and feet dull pinkish red; iris dark brown, with broad crimson eye-ring. In flight, shows broad triangular white patch in centre of wing. Non-breeding adult lacks dark hood but retains dusky orbital ring. Juvenile blackish brown above with pale feather edges giving scaly appearance; wing pattern similar to adult but forked tail has narrow, greyish terminal bar, with distinctive broad dark patch around eye and there smaller dark ear-spot. First-winter resembles juvenile but back is grey (1 Olsen, K. M., and H. Larsson (2003). Gulls of Europe, Asia and North America. Christopher Helm, London, UK. Close ).

Galapagos, mainly in E half of archipelago; also Malpelo I, off W Colombia. Disperses offshore along South American coast from Ecuador to C Peru, occasionally to C Chile.

Virtually endemic to the Galapagos Is, where nests on all major and several minor islands, but avoids the colder waters around E Fernandina and W Isabela. Breeds mainly on steep slopes or broken cliffs, often on broad clifftop ledges, but also just above the wave zone; also on gravelly beaches and under vegetation. Many nests are readily accessible to humans. Fairly pelagic, and quite often observed feeding 500 km from nearest land.

Adults leave colony after breeding, but return in 4–5 months, often to their previous nest-site; young never return to same colony. From Galapagos main movement is E. Highly pelagic during non-breeding season, but often seen off South American coast from Ecuador to C Peru , occasionally to C Chile (33° S). Records off Pacific coast of Costa Rica suggest regular presence there, at least during March–May (2 Young, B.E., Easley, K., Garrigues, R., Mactavish, B., Murgatroyd, P. and Zook, J.R. (2010). Swallow-tailed Gull Creagrus furcatus in Costa Rica Cotinga. 32: 24–26. Close ). Accidental in Panama and Nicaragua; recorded twice in USA (California, in March and June) (3 Howell, S. N. G., I. Lewington, and W. Russell (2014). Rare Birds of North America. Princeton University Press, Princeton, NJ, USA. Close ).

Feeds mostly at night, but also by day. The most nocturnal of all gulls, with associated and characteristic large eyes. The ability to be active mainly at night but also when necessary in daytime has an endocrine basis, the species showing no measurable daily melatonin rhythm unlike diurnal gulls (4 Wikelski, M., Tarlow, E.M., Eising, C.M., Groothuis, T.G.G. and Gwinner, E. (2006). Do night-active birds lack daily melatonin rhythms? A case study comparing a diurnal and a nocturnal-foraging gull species Journal of Ornithology. 147: 107–111. Close ). Exploits squid heavily at night, when latter more readily available near surface; also takes clupeid fishes. Regurgitated samples included Sardinops, as well as squid (Symplectoteuthis oualaniensis). Appears to suffer from periodic food shortages.

Range of calls thought to function in echolocation; most frequent call is a rattle interspersed by a piercing “pee”.

Breeds throughout year; very asynchronous across Galapagos , but individual subcolonies are synchronized by social interactions. Breeding cycle generally lasts c. 6 months, but 9–10 months for individual pairs. Forms loose colonies with large inter-nest distances; densest colony contained 30 pairs in 200 m²; often solitary. Nests on soil, bare lava or gravel, often under rocks and bushes; most nests close to sea. Slight hollow, lined with pebbles, feathers or bones; adults may swallow pebbles and regurgitate them at the nest. Lays single egg, pale greenish or bluish white with large brown spots, size 60–66 mm × 44·5–47·5 mm; incubation 33–35 days, by both sexes, with shift sometimes lasting c. 24 hours; eggshells not removed; chick brooded continuously for first 48 hours, and remains close to nest for several weeks; both parents almost invariably attend nest during daylight; fledging 58–65 days, but young remain near colony and dependent on parents until 90–140 days. The single-egg clutch (and brood) may be related to the pelagic lifestyle, with its often infrequent provisioning rate. Nevertheless, artificially doubled broods enjoy a higher fledging success so it is suggested that it is the cost of producing two eggs that limits clutch size to one egg (5 Agreda, A. and Anderson, D.J. (2003). Evolution of single-chick broods in the Swallow-tailed Gull Creagrus furcatus. Ibis. 145 (online): E53–E58. Close ).  

Not globally threatened (Least Concern). Total population c. 10,000–15,000 pairs in more than 50 colonies on all major islands; 50 pairs on Malpelo I, Colombia. Currently protected by Galapagos National Park, but increasing human population and a burgeoning fishery are causes for concern.