Here's the three Long-billed Dowitchers.
The bird on the right is basically callable based on the bill alone which is saying something since there's a lot of overlap between the two species. These birds are pretty close to full breeding plumage (there still are some wing coverts coming in, and the birds aren't fully red back to the rear-most flanks). There is more than enough warm coloration, though, to know that the only candidates are Hendersoni Short-billed Dowitcher (the subspecies we get), or Long-billed. These pics aren't the greatest; once I get a digi-scoping camera back this would been a good chance for Pepsi challenge photos between full-frame digi-scope and these heavy crops from the 300mm lens. However, they do show enough of the upperparts to evaluate one of the easier ways to separate the two in spring. Long-billed has dark scapulars with some fine rufous markings and some contrasting white tips. Short-billed has broader gold markings in the scapulars.
Honestly, I'd bet you could guess the ID of dowitchers in Michigan at about a 95% clip based on dates alone, the first week or so of May are Long-billed. The 3rd week of May is Short-billed. In the fall, July and August are Short-billed, September could be either, October is Long-billed.