The two swallows I see most often in Northfield are tree swallows and barn swallows. Tree swallows are recognizable by their white undersides and iridescent blue-green backs. Barn swallows have a reddish-brown underside with a steel-blue back. Both are beautiful birds that are a great pleasure to watch as they swoop through the air catching insects.
Tree swallows like to nest in bluebird nesting boxes, and often beat the bluebirds to it. That is why people often put up two boxes near each other. If there is only one box, the tree swallows get there first and the bluebirds are out of luck. If there are two boxes, there will be one left for the bluebirds because a tree swallow will tolerate a bluebird next door, but won't tolerate another tree swallow so close.
These photos were taken at the St. Olaf College nature area. The bird above, sitting on a fence post at the edge of the path near two nesting boxes, let us get surprisingly close to take this photo. Below, two swallows out of a group of five or six are seen swooping around a different set of nesting boxes.