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 has not been quantified, 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.
The global population size has not been quantified, though national population estimates include c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in China, c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration in Taiwan and c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in Japan (Brazil 2009).
The population trend is difficult to determine because of uncertainty over the extent of threats to the species (del Hoyo et al. 1996).
This species ranges in tropical and subtropical areas of the Indian Ocean and western Pacific Ocean. In the western Indian Ocean it breeds on the Aldabra and Amirante Islands, Seychelles, Chagos Islands (British Indian Ocean Territory) and the Maldives and can be found on the eastern African coast. Its range in the eastern Indian Ocean and Pacific ecompasses the Andaman Islands, India, east to southern Japan and China, south through Indonesia, Malaysia, the Philippenes and New Guinea to north-east Australia and some islands in the western-central Pacific (del Hoyo et al. 1996).
This species frequents small offshore islands, reeds, sand spits and rocky cays, feeding in atoll lagoons and close inshore over breakers, but sometimes also at sea. It feeds mainly on small fish and will almost always forage singly by shallow plunge-diving or surface-diving. Its breeding season varies depending on locality, usually forming small colonies of 5 to 20 pairs, but sometimes up to 200 pairs. Colonies are often monospecific and formed on unlined depression in the sand or in gravel pockets on coral banks close to the high tide line (del Hoyo et al. 1996).
This species is extremely sensitive to disturbance, with relatively little disturbance needed to cause nest desertion (Department of the Environment 2018); however, most breeding locations are in remote locations, so this represents a relatively small threat to the species. Is prone to predation by invasive rats, which cause considerable losses due to chick and egg predation, and can lead to breeding failure (Department of the Environment 2018).
Text account compilers
Ekstrom, J., Butchart, S., Martin, R., Stuart, A., Calvert, R.
BirdLife International (2020) Species factsheet: Sterna sumatrana. Downloaded from http://www.birdlife.org on 30/09/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 30/09/2020.