Easter mystery?

We have a flower box on our front windowsill. It had dead plants in it from last summer for a while, but last month I finally re-planted it with spring flowers. Since it's right there, I check it as I leave in the morning, dead-head the spent flowers, quickly make sure it's not too dry, etc.

Today, I noticed that one part of the planter had soil that seemed to have been stirred up or disturbed. I figure it was birds or maybe the white cat that hangs around- no big deal. One of the young petunias was unearthed, so I dug around to put it back into the dirt. And found something weird and hard just under the top of the soil.

Eh? Scraped around a bit, and uncovered enough to see that it looked like an egg. By this time, I had to rush to catch my train, so I asked The Limey to check it out. It was an egg all right. But what an egg!

I've put it here next to a normal chicken egg. It's huge!

Maybe a goose egg? How'd it get in my planter? Easter egg hunt hijinks? Who'd dig up my petunias to hide it? Is it still good? Can we eat it? Was it fertilized?

We'll never know.