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.
The overall population trend is stable, although some populations have unknown trends (Wetlands International 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 Mexico1.
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).
Text account compilers
Ekstrom, J., Butchart, S., Calvert, R.
BirdLife International (2017) Species factsheet: Gygis alba. Downloaded from http://www.birdlife.org on 18/10/2017. Recommended citation for factsheets for more than one species: BirdLife International (2017) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/10/2017.