I thought these were probably females or immature birds and asked for help deciding between the two options. Ooops! A couple of MassBirders suggested that these were probably "eclipse" males (a) because the males are in this plumage at this time of year, and (b) because of the amount of white in the wing as seen in the next photo (check this out in Sibley).

After reading the detailed article in Cornell's Birds of North America Online, I learned that the males migrate in August and the females and young birds go much later than the males, thus the August 19th date confirms what the strong presence of white in the wing shows.... these must be males in the Basic or eclipse plumage.

Taken on Duxbury Beach (MA) 9:07 am August 19, 2009