Well the OP may not have been comfortable with using a blow to the head to kill them as for some people it's just too traumatic (no pun intended) and they are unable to bring themselves to do it.
Asphixiation is commonly used when people can't bring themselves to kill by "violent" methods.
So i wouldn't go so hard on the OP.
You also have to wonder whether the immature birds even have the ability of cognitive thought, be able to feel pain and be fully aware of their situation and surroundings. In which case they wouldn't have suffered as their brain couldn't comprehend as it's just not capable of doing so.
I'm not on either side just thinking out loud