In the attached screenshots my experiment was done in creative mode but also capably done in survival mode in the current snapshot 18w30b in which baby chickens spawned by using standard eggs (not spawn eggs) stay airborne, they still have hit boxes and can be killed, the main way I found to fix this is to return to the title screen and rejoin world, the problem I have is it makes it absolute garbage for automatic chicken farms as since the never stop floating they never grow up.