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.