An interesting, if normally hard to see field mark is visible in this distant shot as these birds worked around the area. Notice, that on most of the birds, the unmarked white lesser coverts of the underwing really stand out from the rest of the feathering. The extent of this varies between individual birds, but can be seen to some extent on most of these birds that have the right angle. This would let you know that at least the bulk of this flock is Long-billed, as the lesser coverts of Short-billed are uniform with the rest of the underwing, so that contrasty patch would be missing.