20 April 2011 and 11 August 2011. Because they compete with Eastern Bluebirds for nesting holes, the swallows are sometimes maligned by bluebird enthusiasts. If you favor bluebirds over Tree Swallows, you have a number of options. Keep your bluebird boxes widely separated. Bluebirds defend larger territories than do swallows. Too many boxes increase swallow/bluebird aggressive encounters. Bluebirds often are the victors when there are not too many swallows in bluebirds' habitat.
Surprisingly another solution is to pair bluebird houses. Swallows will drive out close-by swallows, but they often ignore nearby bluebirds. Even back-to-back boxes will work. Nevertheless, the houses for bluebirds should still be widely (300 feet) separated. These bluebird facts are from Chris Gates and his website that makes for interesting reading. This year my friend Penny has been having a great time running a bluebird trail--check out her website.