What's killing, partially eating birds in my outside birdhouses?

Question:This summer, I am having problems with one birdhouse location. The young have been on the ground dead/bloodied on the head. I just removed a headless adult body from the nestbox. There were no innards so I don't know if the maggots got it or it was eaten. There are large maggots underneath where the nest was. It was an English Sparrow nest in an Eastern Bluebird house. It has a latch door that I can open. We have had the nestbox in the same area for years and no problems until this year. There are other active nestboxes nearby, the same as in the past. Please offer some useful advice. We do not let our pets roam. They are indoor animals.

No joking answers please.
Would wire screen over the front of the nestbox help any?

Answers:
I would suspect a larger bird of prey like a hawk now residing in the area . Perhaps the screen may help if just enough room for the smaller birds to enter would be helpful.