Swallow-tailed gull

Creagrus furcatus is a gull that is near-endemic to the Galápagos Islands ^[1]. According to its authors, it is the only gull that forages exclusively at night: across 114 GPS-tracked trips from 4 colonies, nearly all time at sea (99.5%) fell in night hours ^[1]. The IUCN lists it as Least Concern ^[2].

The eyes of the species have been measured in detail. Compared with other gulls, it has a slightly larger cornea and a longer eye, but its transverse orbit diameter and eye shape do not differ significantly from those of other larids ^[3].

A reflective layer, the tapetum lucidum, has also been described; it sends light back through the retina and increases the light available to the photoreceptors ^[4]. At the colony the bird is absent between dusk and dawn and spends the day on or near its nest ^[1].

The species is considered near-endemic to the Galápagos archipelago ^[1]. The Galápagos lie about 1000 km from mainland Ecuador ^[5].

In a GPS study on 3 islands and 4 colonies, the Genovesa colony held more than 1000 breeding pairs, South Plazas between 500 and 1000, and Española more than 2000 ^[1].

Foraging trips took place mostly over waters shallower than 2000 m, on the Galápagos platform and its slope, and colonies had separate foraging zones with no overlap ^[1].

Pairs breed asynchronously in 9-month cycles, so reproduction is not seasonal at the population level ^[1]. A breeding attempt lasts about 120 days on average ^[4].

It lays a single egg and so raises a single chick. In a brood-doubling experiment with 50 enlarged broods, none raised more than one independent offspring ^[6].

Adults spend the day at or near the nest. In 114 trips by 114 birds, departure was typically around dusk, near 18:00, and arrival before dawn, near 06:00; 113 trips were completed within 11.7 h ^[1].

The diet consists mainly of the squid Sthenoteuthis oualaniensis and of clupeid fish; these prey move up in the water column after dark, and the bird catches them by plunging at the surface and can reach only the top 1 m of the water column ^[4].

Time spent on the water at night changes with the moon: it was close to zero at full moon and reached up to 49% at new moon, in the example illustrated with three birds ^[4].

During breeding, night trips reached between 19 and 103 km from the nest; about 25% of the time at sea was commuting and about half was area-restricted search ^[1]. By day, frigatebirds chase these gulls to steal their food, but they do not do so at night near the colony ^[4].

Globally, the IUCN lists the species as Least Concern ^[2].

Most foraging trips took place inside the Galápagos Marine Reserve, which underlines the importance of this protected area for the species during breeding ^[1].

The tourist and resident population of the Galápagos grew about 5 times in 25 years, and many exotic species have been introduced; there is no evidence that introduced diseases affect this gull ^[5].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

No data on toxins or risks to people are available here. A health baseline does exist: blood was sampled from 58 adults, 28 at Islote Pitt and 30 at South Plaza, all deemed clinically healthy ^[5].