Blowflies were my first thought too, Lee.  In addition to what Rich and
Kevin have suggested, the bluebird resources I've read suggest actually
checking the nestlings themselves for larvae, especially under the wings,
and removing them if you find any.

By the way, don't confuse blowfly larvae, which are the parasites in
question, with maggots or other fly larvae you'll likely find if the
nestlings were dead for any length of time before you found them.

Here's a website with a lot of information about this problem, and some
photos.

http://www.americanartifacts.com/smma/per/b5c.htm

The author actually started doing nest changes at 6 days after hatching as a
matter of course after losing a number of broods to blowflies, and says that
solved her problem.

I'm really sorry, that's a rotten thing to have happen. Hope you're able to
get to the bottom of the problem.

Best,

Miriam Lawrence
Monkton