Justification of Red List Category
This species has an extremely 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 is extremely large, and hence does not 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.
The global population is estimated to number c.35 million individuals (Wetlands International 2020), which roughly equates to 23 million mature individuals.
The overall population trend is uncertain, as some populations are fluctuating, some are stable and others have unknown trends (Wetlands International 2020).
Sooty Tern breeds on tropical islands and ranges through most of the tropical oceans (del Hoyo et al. 1996).
Behaviour The species is dispersive and migratory (Higgins and Davies 1996). At most colonies, adults leave for the open sea after breeding and become strongly pelagic for 2-3 months before returning to the breeding grounds (del Hoyo et al. 1996, Higgins and Davies 1996). On returning to the breeding colonies, they continue to forage pelagically by day and settle on land at night for a further 2-3 months before beginning to breed (del Hoyo et al. 1996). The timing of breeding varies throughout the species's range, with nesting occurring all year round at some colony sites, but seasonally at others (del Hoyo et al. 1996). The species nests in very large colonies (del Hoyo et al. 1996) and remains gregarious throughout the year (congregating in small to large flocks to feed at sea), although it may be observed singly (Higgins and Davies 1996).
Habitat Breeding It breeds on flat, open, sparsely or heavily vegetated, oceanic or barrier islands of sand, coral or rock in productive tropical and subtropical offshore waters rich in plankton, fish and squid (del Hoyo et al. 1996). It is absent from cold current areas and generally avoids islands with terrestrial predators (del Hoyo et al. 1996). Non-breeding Outside of the breeding season, the species is highly pelagic, but generally avoids cold current areas (del Hoyo et al. 1996).
Diet Its diet consists predominantly of fish up to 18 cm long (usually 6-8 cm) and squid, but it also occasionally takes crustaceans, insects and offal (del Hoyo et al. 1996). The species is reliant upon prey driven to the surface by predatory fish (e.g. tuna, Scombidae), especially when breeding (Higgins and Davies 1996).
Breeding site The nest is a slight depression or scrape on the ground (del Hoyo et al. 1996, Higgins and Davies 1996). It shows a preference for nesting on flat, bare sand, coral grit or shell amongst low vegetation on beaches above the high-water mark or on coral islands, atolls and sandbanks (del Hoyo et al. 1996, Higgins and Davies 1996). Less often, it may nest on rock stacks or other offshore islets, and on ledges or terraces of cliffs (although it avoids sheer cliff-faces) (Higgins and Davies 1996). It nests in dense colonies, within which neighbouring nests may be placed c.50 cm apart (del Hoyo et al. 1996). Higher nesting densities occur in areas where bare ground predominates but which are vegetated with plants greater than or equal to 15 cm tall (Feare et al. 1997).
Some colonies (e.g. Ascension Island) (del Hoyo et al. 1996) are threatened by predation from introduced rats Rattus spp. and domestic cats Felis catus (Rodriguez et al. 2006), and the species has been displaced from nesting colonies on Bird Island by invasive ants Anoplolepis longipes (Feare 1999). Colonies have declined in line with observed increases in sea temperatures in Australia, in contrast to the fortunes of Bridled Tern, which have formed new colonies further south (Dunlop and Surman 2012). Variations in sea-surface temperature have been shown to negatively influence the species's foraging success at breeding colonies, hence lowering reproductive success (Erwin and Congdon 2007). Previously, persistent large scale egg-collecting in some areas has encouraged the species to move to suboptimal nesting sites, resulting in higher mortality and reduced reproductive success (van Halewyn and Norton 1984).
Conservation Actions Underway
The mortality of the species was significantly reduced on Isla Isabel (a densely forested tropical island off the Pacific coast of Mexico) by the eradication of introduced domestic cats Felis catus using a combination of poisoning, trapping and hunting with firearms (Rodriguez et al. 2006). The nesting density, and therefore the number of breeding pairs, was increased in colonies on Bird Island by clearing areas of dense native or introduced vegetation and burning existing colony areas annually to prevent the growth of woody plants (Feare et al. 1997).
Conservation Actions ProposedPrevent the spread of invasive predators around breeding colonies.
Text account compilers
Bennett, S., Butchart, S., Calvert, R., Ekstrom, J., Malpas, L., Martin, R. & Stuart, A.
BirdLife International (2021) Species factsheet: Onychoprion fuscatus. Downloaded from http://www.birdlife.org on 26/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/10/2021.