Why do you think they are tibetana, excluding northern breaders?
We believe that the geograhical variation of Common Tern across Asia is reasonably correctly presented in the BWP update. So, some of the breeding birds as far west as the Ob basin have a blackish bill, as shown here:
http://www.elisanet.fi/antero.li ... irat/hir_aasia.html
for example birds 9, 11 and 12.
The variation is largely clinal, and minussensis is a name inside this cline. That means that giving precise subspecific names in the wintering and migration areas is difficult. Tibetana may lie outside this cline, it should have a distinctly darker mantle than longipennis, but mostly red bill in summer, so it should be close to the nominate in this respect.
