Brown-headed cowbird

Somehow a cowbird managed to deposit her egg in one of my bluebird boxes. Cowbird eggs require less incubation time and the cowbird chick, which is larger than bluebird chicks, will hatch first. It then competes for food with the bluebird babies. Oddly, when I checked the box after about 12 days, after seeing Mama Bluebird fly away, I found only the cowbird chick and the bluebird eggs weren’t even there anymore. I saw no bluebird eggs on the ground or pieces of shell in the box, so this is a mystery for me.