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LC
Grey-backed Tern Sterna lunata

Justification
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.

Taxonomic source(s)
AOU. 1998. Check-list of North American birds. American Ornithologists' Union, Washington, D.C.
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.

Taxonomic note
The BirdLife Taxonomic Working Group is aware that phylogenetic analyses have been published which have proposed generic rearrangements which may affect this species, but prefers to wait until work by other taxonomists reveals how these changes affect the entire groups involved.

Synonym(s)
Onychoprion lunatus AOU checklist (1998 + supplements), Onychoprion lunatus

Distribution and population
The Grey-backed Tern can be found in the tropical Pacific Ocean, from the Hawaiian Islands (USA) and Northern Mariana Islands (to USA) 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).

Trend justification
Although Wetlands International consider the population trend to be unknown, the population is suspected to be in decline owing to predation at breeding sites (del Hoyo et al. 1996).

Ecology
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).

References
del Hoyo, J.; Elliott, A.; Sargatal, J. 1996. Handbook of the Birds of the World, vol. 3: Hoatzin to Auks. Lynx Edicions, Barcelona, Spain.

Delany, S.; Scott, D. 2006. Waterbird population estimates. Wetlands International, Wageningen, The Netherlands.

Text account compilers
Ekstrom, J., Butchart, S., Calvert, R.

IUCN Red List evaluators
Butchart, S., Symes, A.

Recommended citation
BirdLife International (2014) Species factsheet: Sterna lunata. Downloaded from http://www.birdlife.org on 17/04/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 17/04/2014.

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Key facts
Current IUCN Red List category Least Concern
Family Laridae (Gulls and terns)
Species name author Peale, 1848
Population size mature individuals
Population trend Unknown
Distribution size (breeding/resident) 37,400 km2
Country endemic? No
Links to further information
- Additional Information on this species