This Eurasian wader has undergone severe declines, particularly in Britain and is listed as "Near Threatened" on the IUCN Red List. This is the Palearctic equivalent of the Hudsonian Godwit (L. haemastica) which replaces it in the Americas. They can be distinguished in all plumages by their underwing patterns, white in Black-tailed and mostly dark in Hudsonian. However there are other differences. The bill is straight on Black-tailed, not noticeably upturned. Also Black-tailed has a broader, longer white wing stripe and broader white tail band (Hudsonian has less white in its tail). Both features can be seen in this flight shot.
Three subspecies are recognized, two of which occur in India in the winter. This is presumably the widespread nominate race. The eastern Black-tailed Godwit (L. L. melanuroides) winters in Assam and Bangladesh west to West Bengal and Odisha where it overlaps with nominate L. l. limosa in winter. It is slightly smaller and darker than nominate and has a smaller wing-stripe more like Hudsonian. It also has a smaller bill, and smaller but more contrasting supraloral stripe. In all features, our bird best fits the expected nominate race. Some authors including Rasmussen & Anderton split these races into separate species. However a study by Höglund et al. found only minor genetic differences. Canon PowerShot SX50 HS.
References:
Höglund, J., Johansson, T., Beintema, A. & Schekkerman, H. (2009) Phylogeography of the Black-tailed Godwit Limosa limosa: substructuring revealed by mtDNA control region sequences. J. Orn. 150(1): 45-53.
Rasmussen & Anderton. 2012. Birds of South Asia: The Ripley Guide. Lynx Edicions.