Tree Swallows are common in our area in the summer.  They often use our Bluebird nesting boxes to set up housekeeping.    They like to nest in hollow spots in dead trees, but those are not always available.   Living in the country beyond city ordinance and the eyes of neighbors,  we have left an old dying apple tree on our lawn.  It is amazing what this old tree attracts.  Eventually I will have to take it down, but right now it is great wildlife habitat.