Corvids are pretty darned clever...but you might try leaving a radio on & tuned to a talk radio station in your barn. Maybe the sound of human voices will help keep them out.

If you really want to find out who or what is stealing the eggs (I assume there is no 'broken egg' evidence left behind?), set up a camera or video recorder aimed at the nesting area.

Velvet ~:>

Can you leave the dogs in the barn to scare the birds?

You can take the eggs as soon as they are laid and return them when she is ready to incubate. Also, you can build a shelter around the nest.

I have tried building a shelter around the nest, but the magipies just walk right in. The magpies will be nesting soon and won't be around quite as much. I will try to gather an egg or two here and there until I have about 20 and then see if she will sit on them. Last year, I had one female that decided to sit on them and then the other two females joined in.

You're sure it's not a snake or possum?

I'm pretty sure its neither of those. We don't have possums here at all, and we have very few snakes. I have only seen a small garter snake years ago. We are at a pretty high altitude here, which discourages a lot of snakes.