Ken, sounds like the work of a house wren (howr) to me. I had trouble with them a few years back and ended up taking down my boxes because the wrens were everywhere. Even if they weren't planning to use a cavity for nesting, they'd still fill 'em with sticks. They're also quick and very sneaky when doing their damage.
So sorry for your loss. Maybe relocating the box a bit and adding a wren guard would help.
Kathy

Ken, I'm sorry, too. I agree with Kathy, definitely sounds like the work of a house wren to me. I also agree with her advice about moving the box. From what I've read, once a HOWR finds the entrance hole, they don't forget about it and can get by a wren guard if the nest box is in the same location.

My area, in the last 2 to 3 years, is also having to deal with them and it's sad to see the destruction they can and will do. Even if they don't want that particular box, they'll often times remove eggs or young nestlings just for the sake of keeping others from using a box. I used a wren guard successfully on my back yard bluebird box after my brown headed nuthatches started laying eggs in the box. The day I found the first egg I added the wren guard and did not remove it till the BHNU were over 16 days old. They are tiny and fledge on about the 19th or 20th day so I kept it on as long as possible. Since I'd never seen a HOWR at that box, the BHNU were lucky and the wren never found the entrance hole.

As far as them being "quick and very sneaky" we saw a video last year of HOWR removing 4 bluebird nestlings in TWENTY SECONDS flat!

Sorry to hear this Ken. I know losing eggs is not a happy thing, but at least they were not babies tossed. Now you know you have a Wren problem too so you can take steps to keep the next batch safer. There seem to be way more Wren attacks this year than I remember in the last few.

Sounds like you really have some challenges. You have my sympathies!
A tiny poked hole is more likely a house wren's thin sharp beak, sometimes taking them some distance away, or simply tossing them out. The house sparrow will toss the eggs out, but their finch-like beak would not as likely make a tiny poke hole. And the HOSP is more likely to stick around, harass, or even take over the cavity.
"Do BB's destroy their own eggs" ... No
"the nest possibly could have been flooded?" ... are there leaks that could be fixed?
"Are the HOSP really sneaky" ... yes, but HOWR are sneakier ...
"and resistant to the Sparrow Spooker?" ... yes, and house wrens will not be deterred by a sparrow spooker.
House Wrens are even sneakier, faster and silent, leaving no evidence behind, only destroying eggs or young. You might hear them singing, but might never, ever see them! There are some links to sound files of house wren song and chatter on Bet's web pages, so you can learn to hear the unwelcome sounds of house wrens. Visit Bet's web page for more info.
And by the way, the Carolina Wren is a very different bird, welcome almost anywhere; not a threat to other birds.
BlueBars

Here is a link that might be useful: http://www.sialis.org/index.html

Thanks for your comments, guys. I am now going to put up a wren guard--hopefully that will solve the problems.

If I have never used a wren guard before, should I make a large one that covers most of the front of the box, as well as the sides? Or should I make a smaller one first--not covering the entire front or the sides?

Personally I was thinking I should try the smaller one first, since the BB's may not be accustomed to a wren guard.

Ken, I received your email and am trying to locate photos of the wren guard that I made from a Cheerio's cereal box. I did cover the entire front, about 4 inches down completely covering the entrance hole. I only took the sides back about 2 inches on each side and about 3 inches long. I'll send those photos from my other bluebird forum soon as I can find them.

Ken Sure sounds like a wren to me and I've got loads of them here. I'd use the sides and make them go all the way back and I always make my guard long in the front and the birds always figure it out even if it's a new pair.
Janie Gin has my pics somewhere of the cheese-it guard. I'll have to post them on my images on the other site. I still don't know how to post them on this one.
Cathy

Ken,
Yes!!I agree with all the others.I had the same problem 3 yrs ago.That year the Wrens chased my Bbs out of my yard until the end of August.They had to deal with one failed nesting after another.finally,I figured out how to place a wren-guard.Then in August(3rd week) they had a successful nesting!!!
michelle

Janie, if he doesn't have a Cheerio's box, can he use a Rice Chrispie's box?

: )

Elly, LOL! Yep, ANY cardboard box will do!

Cathy, Tammy found the photos from your thread and I emailed them to Ken this morning.

What's all this talk about CEREAL boxes? Everybody knows that only a Cheezit box will work! ;) LOL

Seriously, Ken, use Cathy's design. I used one for my blues who fledged a week and a half ago. There's another one on my TUTI box right now. It will last through all kinds of rain and wind. As long as a wren doesn't already know where the entrance is, you're set and so are the birds.

I really appreciate all of the help that I have gotten so far on this thread. Thank you all who have helped me.

I'm not a macho bluebirder yet, so I'm kind of new to this kind of stuff. I really enjoy bluebirds, but unfortunately, we live in a neighborhood with a dense population of those pesky HOSP's and HOWR's. It's been a few years since we've had a successful nesting of bluebirds, but I'll do anything to change that :).