Someone posted a pic of what they thought was a bluebird fledgling but it turned out to be a cowbird fledgling that had been in the bluebird house with the bluebird nestlings. They had a pic of them before they fledged and you could see two gaping yellow mouths and one gaping red mouth, easily could see the cowbird in with the nestlings. Once the young fledged however the Bluebirds stopped feeding the cowbird. It was following them around and begging but they said the blues wouldn't feed it anymore. I thought that was awesome..wish that always happened when cowbirds invaded nests! Hopefully this is what is happening and they just aren't missing when the adults are feeding it.