Ring-billed Gulls

My neighbor Doug and I have been trying to figure out the birds we've noticed flying in flocks over the fields lately.  We finally decided they were gulls but the pictures we snapped weren't sharp enough to determine the species.  On the way into town I saw a group of them settled down in the middle of a field off the road so grabbed a few shots.  Doug had thought they might be Ring-billed gulls.  Now we just need to figure out if I'm right - that they are gulls that summer at the dump or if Doug is correct - they are gulls migrating through from the Great Lakes area.