There are two babies left in the nest! I didn't see the two until this morning. I'm feeling a lot better about it.
Plus, I did some online research and baby cowbirds do not push their fellow hatchlings out of their nests. They just blend in and get fed with the others. Usually cowbirds choose nests of smaller birds, so that when their baby opens its mouth to get fed, it has the largest mouth and gets fed the most.
So all is pretty good in robinland. It could have been worse.