Swallows can be overwhelming to identify, especially when they are flying around and not sitting still for you to pull out your favorite field guide and study their differences. The other day I came upon this colony of Northern Rough-winged Swallows working on their nest cavities in a bank of dirt. To identify Northern Rough-winged Swallows there are a few field marks are point out below, but the easiest characteristic is that they are really the only plain brown swallow that I know of in my area. They are fun to watch and hard to photograph well.
(click on image to enlarge)
I kind of like how the broken-down lichen-covered fence post and wire are in the frame.