ajohn

The bill is slightly curved but I think difficult to judge due to mud, and the amount of curve is slightly variable in different individuals.

Notice on this bird that the upperparts are streaked. On Black-tailed the upperparts are plain in winter and patterned (orange and black) in summer. This is a good feature, even at a long range.

The bird also looks long-billed relative to short legs and short neck. Black-tailed look longer-legged, longer-necked and more elegant. Again, a good feature for separating these species.

ajohn

Yes, the birds in this latest photo are Black-tailed. Notice how the upperpart pattern looks more spotted with black and orange. Also notice how the bird at the back looks longer-necked than your previous photo. Another thing to notice is that the birds developing breeding plumage have blackish 'bars' on the flanks, whereas the bird in your first photo is developing solid red in the whole of the underparts (although not much red yet).

Black-tailed often stay in a very close flock like this, Bar-tailed are more often seen on their own.