Can someone help us ID this bird?

by Cheri
(Waukon, IA)

I live in NE Iowa, and this summer we have noticed some birds that we cannot identify. We've only seen them flying - the birds have a distinctive white stripe across the width of each wing, underneath, and the overall color seems to be gray. The birds are larger than a swallow. (I am comparing them to swallows because they were out flying around at the same time as the swallows were out.) They also seemed to have the same type of flight as a swallow, but a little more soaring and not so much flapping. I even heard their call one evening, but it is hard to describe. Not really a screech, but it was a harsher, or scratchy tone - not a musical song at all.

We just started seeing these birds about 2 weeks ago and have seen them about 4 times. I have tried the bird identification guides that I have found on the internet, but can't seem to come up with an ID. Can anyone help? Thanks!