Systematics

Neither plumage nor morphometrics vary across the species' range; breeding behavior is unvarying as well. That said, gene flow among breeding populations apparently is slight. Genetic markers (RAPDs-random amplified polymorphic DNA) differentiate birds from Churchill, Manitoba, and Mackenzie, Northwest Territories, with divergence between these two populations several times greater than that for other shorebirds sampled (Haig et al. 1997). Alaska breeders constitute a third population, one closer genetically to the Mackenzie population, but birds from s. and w. Alaska do not differ (Haig et al. 1997).

No subspecies have been described, but L. hudsonica (Latham, 1790) and L. australis Gray, 1848-as well as, evidently, L. edwardsii Swainson and Richardson, 1831, and L. alba Gray, 1847-are junior synonyms of L. haemastica (Linnaeus, 1758).

The genus Limosa (the godwits) comprises a small-there are but four extant species-morphologically and genetically distinct radiation that lies squarely within the family Scolopacidae, the shorebirds. On the basis of morphology and proteins, there are two alternative hypotheses of relationships of Limosa within the Scolopacidae (Higgins and Davies 1996). Studies of plumage patterns of downy young (Jehl 1968a) and protein electrophoresis (Christian et al. 1992) suggest Limosa is closely related to Numenius, the curlews. In contrast, osteology (Chu 1995) supports a close relationship of Limosa and Limnodromus, the dowitchers, places both genera in a clade with calidridine sandpipers and Arenaria, the turnstones, and suggests that Numenius is more distantly related. Chu's (1995) cladistic analysis supports earlier conclusions drawn from studies of mallophagan parasites (Timmermann 1957). More recently, comprehensive genetic phylogenies (Thomas et al. 2004b, Baker et al. 2007) place the genus basal to a radiation of typical sandpipers the includes many familiar genera in the family, such Gallinago (the snipes), Calidris (the "peeps" and stints), and Tringa (the shanks). Numenius is basal to this clade + Limosa.

Within the genus, L. haemastica generally is considered to be an allospecies of L. limosa, the Black-tailed Godwit (Mayr and Short 1970), and these taxa have been classified as conspecific (Johnsgard 1981).

• Bar-tailed x Hudsonian Godwit (hybrid) Limosa lapponica x haemastica
• Black-tailed x Hudsonian Godwit (hybrid) Limosa limosa x haemastica

Little information. Genus Limosa dates to middle Miocene, 15–25 million yr before present (bp), possibly to late Eocene, 40 million yr bp (Brodkorb 1967), with Neogene fossils from California, Florida, and Arizona (Becker 1987a Becker, J. J. (1987). Neogene avian localities of North America. Smithsonian Institution Press, Washington, DC, USA. Close ). No known fossils of Hudsonian Godwit.