On a sad note, in checking the bluebird box today I discovered just three eggs - there were four the last time I checked. I don't see a broken shell anywhere around the nest box and I don't see the female anywhere. The male has been in the crab apple tree on his usual perch but she didn't show up. He's gone now and she hasn't been near the box in over 40 minutes that I know of. OK, she just showed up. I really wish she wouldn't scare me like that and I have no idea what happened to egg number four. One of nature's mysteries.