Justification of Red List Category
This species has a very large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend is not known, but the population is not believed to be decreasing sufficiently rapidly to approach the thresholds under the population trend criterion (>30% decline over ten years or three generations). The population size may be moderately small to large, but it is not believed to approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
Croxall et al. (1984) estimate the population at 35,000 individuals.
The population trend is difficult to determine because of uncertainty over the extent of threats to the species (del Hoyo et al. 1996).
The Swallow-tailed Gull breeds mainly on the Galapagos Islands, Ecuador (all major and several minor islands) but also on Malpelo Island, Colombia. When not breeding it can be found along the Pacific coast of South American from Ecuador to northern Chile (del Hoyo et al. 1996).
This species feeds mostly at night but also by day, heavily exploiting squid at night but also taking clupeid fish. It appears to suffer from periodic food shortages, and is often observed feeding 500 km from the nearest land. It breeds throughout the year and asynchronously across the Galapagos, with individual subcolonials being synchronised by social interactions. It forms loose colonies with large inter-nest distances but can be solitary, nesting on steep slopes or broken cliffs, often on broad clifftop ledges but also just above the wave line, and on gravelly beaches and under vegetation. Adults leave the colony after breeding and become highly pelagic, returning in 4-5 months often to their previous nest site.
Food shortages are noted to periodically impact the species, associated with El Niño events and system wide food shortages within the species's range, resulting in the temporary interruption of breeding cycles (Valle et al. 1987). The staggered breeding timing of the various colonies across the islands reduces the scope of the impact and the population appears to be able to recover in intervening years, meaning this has little impact on the long-term overall population.
Text account compilers
Butchart, S., Ekstrom, J., Martin, R., Calvert, R.
BirdLife International (2020) Species factsheet: Creagrus furcatus. Downloaded from http://www.birdlife.org on 28/11/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 28/11/2020.