Some larger bird species frequently lay one more egg than they can typically raise successfully. This extra egg acts as an insurance policy. Usually this baby is too small to compete with it's siblings. However, if something happens to one of the larger babies, the parents still have a chance to reproduce. If it's a particularly successful year, the parents can raise an extra baby!
The male osprey comes in for a landing
One fledgling on the left, adult female in the middle, and adult male on the right