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 appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very 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.
Delany and Scott (2006) estimate the population at 150000-1100000 individuals.
The overall population trend is stable, although some populations have unknown trends (Delany and Scott 2006).
The Common White Tern has a distribution across the tropics of the world, being found year-round on islands in the south Atlantic Ocean, the Indian Ocean, and the western and central Pacific. It is also a seasonal visitor to islands in the south-central and eastern Pacific off the coast of Mexico (del Hoyo et al. 1996).
This species feeds mainly upon small fish, but will also take squid and crustaceans. The proportions vary locally, with 50% of its diet comprising of fish on Christmas Island compared to 85% on Hawaii. Prey are caught by diving. Its breeding season varies locally, nesting on coral islands, usually with vegetation, nesting in trees and bushes, on rocky slopes and cliffs, and also on artificial substrates. It lays a single egg on a bare branch, usually within a slight depression at a fork or on the midrib of a palm frond or banana leaf. It is resident year round on some islands though is a seasonal visitor on others where its non-breeding movements are not known (del Hoyo et al. 1996).
The White Tern is at risk from the impacts of invasive species, with rats Rattus spp. threatening to extirpate the breeding population on Clipperton Island (Pitman et al. 2005). Cats Felis catus have been observed to predate White Terns (Bell 1912), and they likely predate both chicks and nesting adults. They are known to exist as an invasive species in many areas within the species' range, including Raoul Island where it is thought their eradication would benefit the few White Terns left on the island (Fitzgerald 1991). Pied Currawong Strepera graculina and Masked Owls Tyto novaehollandiae are known predators of White Terns on Lord Howe Island (Carlile and Priddel 2015), and Barn Owls Tyto alba impose a significant predation pressure on Aride Island, Seychelles, but is currently subject to a control programme (Gochfield et al. 2018). Human exploitation also poses a problem in some areas, with the extreme example of a corrupt warden in Rodrigues causing the population on the island to collapse by killing adults and collecting eggs (Gochfield et al. 2018).
Text account compilers
Calvert, R., Butchart, S., Martin, R., Stuart, A., Ekstrom, J.
BirdLife International (2020) Species factsheet: Gygis alba. Downloaded from http://www.birdlife.org on 20/02/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 20/02/2020.