What happened to my bird egg?

I have hatched chickens, and I tried to raise baby starlings before. Did you find it on the ground? If you did, more than likely, it was pushed or fell out of the nest. It sounds like it was developing right and died for some reason. Sometimes when something starts to hatch they will die. This would account for the the crack and placenta. Or if it fell out, this would account for the crack as well. The fall could have killed it. Starlings are born almost naked with there eyes closed, so it sounds like it was fully developed. As they begin to hatch, they eat the placenta and cut the shell in two. If you help with the hatching process, they will die. It sounds like you did everything you could. Found eggs are hard to hatch, even if you know they are all right.