Marine life guide of Peru · Laridae
Swallow-tailed gull
Creagrus furcatus (Néboux, 1842)
Species account
Summary
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].
Identification and similar species
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].
Distribution and habitat
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].
Biology
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].
Ecology
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].
Conservation
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].
Health, toxicological and chemical relevance
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].
References (6)
- Cruz, S.M., Halpin, L.R., Proaño, C. et al. (2025). The owl gull: Exclusively nocturnal foraging by the Swallow-tailed Gull Creagrus furcatus in Galápagos. Marine Ornithology 53(1), 63-69. doi:10.5038/2074-1235.53.1.1626 (accessed 2026-10-25)
- BirdLife International (2018). Creagrus furcatus: BirdLife International. The IUCN Red List of Threatened Species 2018: e.T22694493A132556163. doi:10.2305/iucn.uk.2018-2.rlts.t22694493a132556163.en (accessed 2026-10-25)
- Iwaniuk, A.N., Heesy, C.P., Hall, M.I. (2010). Morphometrics of the eyes and orbits of the nocturnal Swallow-tailed Gull (Creagrus furcatus). Canadian Journal of Zoology 88, 855-865. doi:10.1139/z10-051 (accessed 2026-10-25)
- Cruz, S.M., Hooten, M., Huyvaert, K.P. et al. (2013). At–Sea Behavior Varies with Lunar Phase in a Nocturnal Pelagic Seabird, the Swallow-Tailed Gull. PLoS ONE 8(2), e56889. doi:10.1371/journal.pone.0056889 (accessed 2026-10-25)
- Valle, C.A., Ulloa, C., Regalado, C. (2020). Baseline haematology, biochemistry, blood gas values and health status of the Galapagos swallow-tailed gull (Creagrus furcatus). Conservation Physiology 8, coaa064. doi:10.1093/conphys/coaa064 (accessed 2026-10-25)
- Ágreda, A.E., Anderson, D.J. (2003). Evolution of single-chick broods in the Swallow-tailed Gull Creagrus furcatus. Ibis 145(2). doi:10.1046/j.1474-919x.2003.00160.x (accessed 2026-10-25)