Monotypic at Least

The three subspecies of Least Tern in North America have been determined to be invalid, according to new research.

The American Ornithologists' Union recognises three subspecies of Least Tern Sternula antillarum: S a antillarum in the east, S a athalassos in the interior and S a browni in California; S a mexicana and S a staebleri from Mexico are taxonomically uncertain.

A US government team of scientists used mitochondrial and microsatellite DNA data to delineate the genetic structure of the three forms, only to discover that the subspecific groups did not stand in their recognised geographical ranges, and any genetic structure was likely to be due to gender difference s in post-breeding dispersal patterns.

Genetic diversity was relatively high among individuals from all colonies sampled, and microsatellites showed that diversity was largely consistent with genetic drfit among popaultions at some geographical remove.

The traditional subspecies were not supported by the team's analyses, as had been suggested by other previous recent studies. The taxonomic designation of subspecies is used as a basis for conservation legislation in the United States, and Least Tern has undergone a substantial historical decline in numbers, largely due to anthropogenic influences.

Both the Californian and interior populations are currently classified as 'endangered' by the United States Fish and Wildlife Service, as is the eastern population in many individual states. It is as yet not known what effect this possibly unfavourable taxonomic reassessment will have on the conservation of the species, but it is apparent that current legislation may be partly based on fallacy.

Reference: Draheim, H M, Miller, M P, Baird, P and Haig, S M. 2010. Subspecific status and population genetic structure of Least Terns (Sternula antillarum) inferred by mitochondrial DNA control-region sequences and microsatellite DNA. The Auk 127: 807-819.