Sunday, our Yellow-thighed breeding pair ("Mom" and "Pop") had an egg in their nest box. I was thrilled. We've raised four clutches from them, and they've been good parents. Then, yesterday, our Black-head pair ("Bonnie and Clyde") had an egg in their nest. That was more exciting because this is the first egg from this our newest pair! This evening, I checked in, and there was no egg in Mom and Pop's box...nothing, not even a shell. Anyone else have this happen? Will it be a habit for the rest of the eggs in this clutch (as they're laid)? Should we do something to prevent it?