Bluebird Fun House

Readers will recall that two weeks ago I wrote off hopes for a bluebird nest in a bluebird-friendly box-on-a-post. And indeed, there's no sign of an active nest of any kind within. And yet... strange things are going on there. I had seen a male perching on the box, but until today got the idea in my head that he was only resting there. Today, quite by chance, I saw him perch, enter, spend time within, and withdraw. Somewhat later I took a look inside. There's nothing; only this abandoned wren nest. What the heck is going on? Theory #1: the Bluebird is raiding this box for nest material. Rebuttal: the nest is not shrinking in size, and it's a bit too early for a 2nd round nest build. Theory #2: there are great insects inside and the bluebird is hunting them out. Rebuttal: none found when I opened and poked around. It's got me completely stumped. A new behavior... must keep watching for more data...