Kudos to everyone who made a guess at the photo id contest--it was pure evil on my part. The northern rough-winged swallow is not a bird I normally talk about, it's brown and nondescript and just not a bird people pay a lot of attention to--even though they are there and easy to see if you look. A big pat on the back to Leanne for the correct answer.
Here's the original photo of the bird that was zoomed in for the contest. We had two swallows fly into the nets at the same time at Carpenter--near the bird feeders of all places! Since these aren't birds that will come to bird feeders, our best guess is that they were chasing each other over a territory battle and both landed in the nets at the same time.
At first, there was some talk that it might be a swift, but a check of the tail--and running fingers along the wing confirmed that this was a northern rough-winged swallow. The barbs on in the primary wing feathers on a male rough-wing are distinctly hooked and running your finger over them kind of feels like running your finger over a nail file. Females do not have as distinct barbs. I don't think scientists have figured out the reason for the barbs yet--if anyone knows or has an idea, please let me know in the comments. I would have gotten a photo of the barbs, but the birds were flappy and stressed and we wanted them back out on the wing ASAP.
Again, thanks to everyone who guessed. It's not easy to put your name to an id when you aren't sure.