Tachycineta

Tachycineta is a genus of birds in the swallow family, including these two birds, both native to Colorado. Tree Swallow (T. bicolor) is a familiar swallow in eastern North America.

Violet-green Swallow (T. thalassina) is a similar swallow with a western distribution. The colors are different, but that is often very hard to distinguish. Note the white coming up around the eye on the Violet-green Swallow.