- bill color slightly more orange, with yellow base (vs. orange with pale pink base on the other bird); but this is very very difficult to judge and essentially worthless as a field mark (see my post from last fall about judging bill color).
- whole head nearly uniform dark brownish, sometimes looking paler on the cheeks but still with solidly dark crown above the eyes (vs. paler gray-brown head and neck with narrow dark crown stripe and contrasting dark border along white front). This is highly dependent on lighting, and would be difficult to judge on a lone bird, but the Greenland-type bird does look consistently darker, and may show a contrasting dark hind-neck that the other bird doesn’t, but I need to check that in more lighting conditions.
- neck strikingly thick and heavy, no thinner than the head (vs. neck slender, obviously narrower than the head and looking graceful and slim). This is one of the most obvious differences on these two individuals.
- margins of mantle and scapular feathers pale brownish, not strongly contrasting (vs. margins whitish, more strongly contrasting).
- white border on flank feathers narrower and not extending as far forward (vs. broader and longer)
- pale tips on median coverts narrow and not white, relatively inconspicuous (vs. broader, whitish, conspicuous)
- greater coverts without pale edges, and with narrow and inconspicuous whitish tips (vs. obvious narrow white edges and broad white tips forming a conspicuous narrow white band across edge of folded wing). This is another obvious difference between these two individuals.
- black smudges all over the small feathers of the belly between the legs and extending well behind the legs (vs. this area virtually all white). This is separate from the irregular black bands before the legs and up onto the flanks, which are marked similarly in both birds.
- white tips on the tail feathers seem to be narrower on the Greenland-type, but this is very difficult to see in the field.
Here’s another photo of the Greenland-type bird, showing the heavy orange bill, dark and thick neck, and the lack of contrasting edges on the feathers across the shoulders. But none of that is very helpful in a single photo without another bird for comparison, nor is it likely to be very helpful on a single bird in the field.