We decided to take a peek and this is what we found...
See the two different types of eggs? This was a first for me. We spent the evening watching the nest to see what type of bird went in and it was sure enough our bluebirds. Apparently, bluebirds, like some other birds will hatch two sets of offspring in Spring and Summer.
Unfortunately, as I researched why there were two different types of eggs I came to some sad news. The other eggs are parasitic. They are most likely from a Brown-headed cowbird. The cowbird probably crept into the bluebirds' nest and actually destroyed some of their eggs. On top of that, the cowbird will hatch first and require more food as it grows larger than the bluebird hatchlings. Momma Bluebird will unknowingly care for the cowbird hatchlings as her own.
Removing the eggs aren't an option for the honest person since they are protected under the U.S. Migratory Bird Act. It is possible the cowbird will destroy the nest if her eggs are removed as well. I'm also hesitant to remove them because the bluebirds may think I'm messing with their eggs. Daddy Bluebird can be quiet vicious. Which brings up another issue. One of the ways to deter a cowbird is to not visit a nest. Now, I'm left wondering if our baby bird watching is what set up our bluebirds for the parasite.
We'll wait it out and see. And next year not hang out at the birdhouse so often!