Temperatures below 27°F can kill the embryos.
Infertility can be a problem too. Infertility can happen very easily when there are too many toms in with too few of hens. The toms will prevent each other from actually breeding the hens. Even if the toms are separated by a see through fence, they can spend all of their time fighting through the fence instead of breeding the hens.
I try to keep a minimum of 4 to 5 hens per tom.
