Can any animal, cat, raccoon, squirrel, etc. get to the box by jumping, climbing or leaping? I don't remember your setup, or if you have a baffle.
HOSP will also do this, but more likely they would damage the eggs as well, and you might see or hear them around in the morning hours trying to claim the box.
Without actually touching or moving the eggs, can you see any damage?
If the female bluebird is OK, they will probably return, rearrange the nest, and continue (or begin) incubation.
I will keep my fingers and toes crossed for you.
Bluebars

Oh no, that nest looks a mess. I have read that racoons will mess up the nest like that. If that happened over night it could have been a racoon that reached in and got the mom! Hopefully that is not the case though! Is your pole baffled? Good luck, hope you can figure it out and that you see your pair of Blues again soon!

donna

I think a raccoon would have taken every single egg. So would a snake. A house wren or HOSP would have more than likely removed the eggs although sometimes HOSP just build right on top of eggs or even babies they've killed.

It's possible that a new female bluebird has claimed this box and fought with the first female and driven her away. At this point I would just watch the box every change you get and see what shows up.

Thanks for the responses.

We have a baffle... but our yard is small so even with our efforts to space well who knows what could have leapt from where. We too thought a Racoon or snake would have taken the eggs. To be honest, in 16 years we've never seen a racoon - day or night. We know that doesn't mean there aren't any around but we think it unlikely. We had thought about cats, there are plenty of those around but the cat would have had to have got on top of the box somehow and in its condition it may not have withstood the weight/impact.
This box has been here a loonnngggg time hence it's condition - we planned to take it down this season but procrastinated and the Blues beat us to it after having their original box borrowed by the Flycatchers. This is the first time anything has attempted to nest in it (other than a few strands of something placed in there last yr by a chickadee). It's part of the 'scenery' and the 'area' squirrels pay it no attention --- or if they happen to come too close in their travels they are dive bombed by Mama and Papa.

We've pretty well stared at the box all day between the two of us! No sign of either Mama or Papa. We figured if something had happened to one the other would still be around?!?! If Mama was spooked would Papa follow her with a brood about to hatch?
The weather this afternoon has been awful - bad storms and rain so most likely we wouldn't see them anyway but it's stopped now and still no sign -- no one came for their evening meal worms.

We've checked the nest and eggs. The nest is dry, no mites, blow flies etc. The eggs are intact - no damage. There were no feathers/dead birds around in either our or adjacent yards.

With the cooler temps from the passing of the storms we fear the eggs are lost even if Mama should return.

So depressing. We've been scratching our heads all day - obsessing, maybe it's this, maybe that. Hubby's putting the worms out now just in case. Guess we'll leave it a day or so and see what happens. If nothing, we'll take this one down..box and all.

Well.... sad to say absolutely no signs of the Blues. The more we think about it (which of course is driving us nuts) the more we think it was a cat (although we still have trouble figuring out how). Obviously it was 'something' that most likely wasn't interested in the eggs as they're still sitting pretty - and no matter how we try to figure it out we'll never know. Wish we had a nest cam so we at least could see who the culprit was and if anything happened to Mama - i.e. other than her being spooked.

So, no doubt we'll be taking the nest out and most likely the box will be coming down.

The flycatchers feldged today - and of course I missed it (other post). So, we'll clean that nest and who knows, maybe the Blues will venture back to their original spot and try a 3rd time. Think that's wishful thinking but ah well.

We suddenly have a very quiet garden....

The summer is not over. Plenty of time to make changes and hope for more cavity nesters. Take that time to do whatever you can to improve their chance of a successful nesting if they decide to choose your offerings.
And hey, congrats on the flycatchers! What a special treat that must have been.
BlueBars