On Avery Road there was, however, a big flock of shorebirds well off the road including about a hundred lesser yellowlegs, about 40 pectorals, and 2 dowitchers. We see this one in nice early morning light, with fairly brick reddish underparts with some difficult-to-characterize patterning to the underparts with dark scapulars with some internal reddish markings and whitish scapular tips:
I don't have much experience with dowitchers in spring. Probably 99% of the dowitchers I've seen are July and August SB dowitchers (of the central Hendersoni subspecies) at Pt Mouillee.
The pics aren't great as the birds were very distant. Between the 60x zoom of the scope, probably 2-3x on the camera (something I don't usually do), plus cropping, these are probably 100-200x. Even with the best of equipment a person would run out of photons.
None of my pics captured the bill that well, though it's accepted that there is a fair amount of overlap. I was not overly-impressed, though, by the length of the bill. LB dowitcher is supposed to be chunkier than SB, "like it swallowed a grapefruit" is the description in the Karlson shorebird guide. I think that would be more helpful if I'd had more recent comparative experience with them. Obviously this character is better evaluated in the field than in single pictures. It looks chunkier in the first pic than the second.
In terms of general plumage, it's considerably darker than the post-breeding adults I'm used to seeing, but that could probably apply nearly as well to a fresh SB than allow us to jump to the conclusion that it's LB, though I think it shades in favor of LB.
Sibley points out LB as having scapulars with rufous bars and white tips whereas SB has bold gold barring. Karlson describes LB's scapulars similarly, notes bright upperparts of SB, and does show photos that are consistent with Sibley's paintings. The Hayman shorebird guide also illustrates this difference in the paintings (though in the descriptions he describes SB as having broad rufous or rusty-buff or pale cinnamon edges to the feathers - but at least says the edging is broad). Kaufman in Advanced Birding emphasizes that the broad edging to SB's scaps make the bird appear very bright (though gives the color to this edging as rusty). My bird's upperparts, dark with rufous marbling with narrow white tips, I think are definitely in the LB camp.
Of my 4 resources, Karlson, Hayman, and Kaufman all emphasise the pattern of the underparts (Sibley is the only one that gives equal weight to the upperparts).
All agree that Hendersoni SB dowitcher should have a clean "fore"neck without patterning whereas LB has streaking in that region. I'm not sure I've ever seen a definition for "fore"neck. I think the first picture shows some streaking in this region, but it's so distant that I think it's hard to draw firm conclusions.
Hayman and Karlson both state that SB had fairly evenly distributed markings from the sides of the breast through the flanks, whereas LB has denser markings along the side of the breast which are more lightly distributed along the flanks. I think distance makes it hard to argue strongly either way. Sibley and Kaufman note that the patterning of the sides of the breast tends toward spotting in SB and barring in LB. Distance makes the bird look somewhat in-between. When I first looked at the photos I thought it showed spotting along the sides of the breast, but when I looke closer, the marks appear more chevron shaped.
I went back and forth earlier in the day on the ID of these birds. My initial gestault impression in the field was that these looked like long-billed dowitchers, though not the longest-billed I've ever seen. I didn't feel totally comfortable with the call initially given that I don't have a ton of experience with the plumage of spring dowitchers, but after having gone through my resources and compared them with my pics, I now feel that these are long-billed's.
Sept 09 Addendum: here's a Jean Irons photoeassy about this same question.