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 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 population is estimated at 100,000-1,000,000 individuals.
The population is suspected to be in decline owing to predation at breeding sites (del Hoyo et al. 1996).
The Grey-backed Tern can be found in the tropical Pacific Ocean, from the Hawaiian Islands (U.S.A.) and Northern Mariana Islands (to U.S.A.) in the north, south through the Phoenix and Line Islands (Kiribati) to Fiji, and east to the Austral and Tuamoto Islands (French Polynesia). It is also found at sea throughout this area (del Hoyo et al. 1996).
This species has a diet comprised mainly of fish and squid, but it will also occasionally take lizards. It feeds mainly inshore, often with other terns, and takes tiny fish by plunge-diving, and also by contact-dipping or hover-dipping over schools of tuna. It has a long breeding period of February to September, breeding on oceanic islands on low sea cliffs, sandy beaches or bare found on coral or rocky islands (del Hoyo et al. 1996).
This species is at risk from heavy predation by House Rats Rattus rattus on Midway, Kure and other islands (Gochfeld et al. 2018).
Text account compilers
Calvert, R., Ekstrom, J., Butchart, S., Bennett, S., Martin, R.
BirdLife International (2019) Species factsheet: Onychoprion lunatus. Downloaded from http://www.birdlife.org on 24/04/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/04/2019.