Here's the thing
Bank Swallows usually show a breast-band with a dark line down the center of the breast as previously mentioned. The birds in the photos don't appear to have that and I would agree with previous comments that what appears to be a breast band, especially in shot four is in fact the wind ruffling up feathers.
Another field point for Bank Swallow (Sand Martin) is the white throat that contrasts with the breast band...again lacking on these birds, while Rough-winged has a dusky grey-brown throat.
A third characteristic of Bank Swallow is the white of the throat curving beind the ear coverts, which is a dusky color on Rough-winged. Contrary to what you say Alex i don't see it in shot one or the other two with the bird facing the camera. I think it is the strong sunlight creating that effect.
Plumage-tone wise Bank also tends to be greyer above than Rough-winged doesn't it? I don't really see that in the photos, but the strong sunlight makes t difficult to assess the true color.
I also think Bank have a more forked tail than Northern Rough-winged. These birds show fairly shallow-forked tails..but this is prettly subjective.
Another ID clincher would be the underwing and tail, with Bank having paler secondaries and primaries and outer undertail feathers, contrasting with the darker axillaries and inner tail feathers, while Rough-winged is all dark on the underwing and undertail.
Rough-winged also appear to have longer wings than Bank and those birds look pretty long-winged to me.
having said all that I have never seen Northern Rough-winged, only Bank Martin and it really is down t PonyRCR to say whether there were breast bands on these birds and the white auricular area behind the eye...and of course being a Brit with no field experience of Rough-wingeds i am happy to bow to greater knowledge.
If they were Northern Rough-wingeds I wish I'd been there to see them, as like so many mainland US birds I haven't had a chance to see them yet.
In Asia we have similar problems distinguishing Pale Martin, Plain Martin and Bank Martin..or at least I do..especially a bird last year on an offshore island in the Sea of Japan called Mishima where Bank Swallow are common, but I suspected that for a long while some are Pale Martin (common in Eastern China but not yet proved in Japan) and then one bird we found last year, but couldn't photograph was actually Plain Martin with only 2-3 records in Japan. Didn't even consider Rough-winged which is unlikely on range of course, but looks very similar to Plain, excet it lacks the contrst between pale rump and mantle of Plain! Also may have got Plain and Pale names mixed up!
Nice pics anyway.
Sean
Hiroshima