Three eggs taken from bluebird nest?

[FishkaFishka]

Hello all,

hopefully this is an appropriate place to post this. I have bluebirds in one of my nest boxes. There were 5 eggs in the box as of two days ago, which had just been laid. Today when I checked the box, there were only two. Then I spotted the remains of three broken eggs on the ground near the box. The parents are definitely still in the area and attentive.
 

I’m not sure how this could have happened. This is the one box on my property that doesn’t have a predator guard yet (so of course they chose that one). But if a predator got in, it seems strange that it would leave two eggs intact. There are wrens who just began building in a box nearby, but I’ve actually never had much trouble with wrens before, despite them being common around here. Is it possible that the bluebirds recognized an issue with eggs and threw them out themselves? 
 

anyways, my main question is should I install a predator guard or wren guard, and if yes is there a way I can make a makeshift one right away with materials I have around my house. 
 

Thanks.

[MichaelLong]

This is a natural process, despite how sad it may be I would not interfere. 

[Charlie Spencer]

6 hours ago, MichaelLong said:

This is a natural process, despite how sad it may be I would not interfere. 

If the nests were in natural locations, I might agree.  Since nest boxes are involved, the 'natural process' has already been assisted.  With all the other disruptions humans have caused, a predator guard or two will hardly even the score.

[jeffsonlinepersona]

I built a flicker box and over the winter put a camera in. We had 2 chicks last year and a pair has returned this year. 4 days ago, the female laid the first egg, but when I checked later, it was missing. The next day another egg, but this time I caught the male removing it on video!

There haven’t been any more eggs. I don’t know what’s going on here. 
 

(photo from video of the bugger taking an egg)