With regard to the tree swallows: I noticed a difference this year, too, at first. The bluebirds came back and got completely settled (instead of bickering for days over real estate with several pairs of swallows). Then one day about six swallows swooped by and dive-bombed the bluebirds. Then nothing. Now the swallows are happily feeding babies in their box down at the pond as usual, and another pair is - for the first time - in a box in our meadow. The boxes usually occupied by the swallows are now home to a House Wren pair and a Great Crest Flycatcher pair! And odd summer.