Congrats, Philly Blues – once they get started in an area they usually come back, but that is just my experience – keep up the good work.
P.S. I had NEVER even seen a bluebird until a neighbor moved across the street from me moved from the country and I swear her bluebirds followed them. She had bluebird boxes out and I saw blues for the 1st time. Well, during the winter we built 2 or 3 boxes, put them out in January or February and have had nests every year since then, which was about early 2007 or 2008.