This thread got me to thinking... Piggies leave a pile of meat when they die, why not butterflies and chickens too? Not a pile of meat, but a pile of milk and a pile of grain or something else entirely.
Amendment to suggestion
I suggest that all animals leave behind something when they die with the possibility of leaving behind an egg of the appropriate kind.
Pigs = Meat + possibility of Piggy Egg
Butterfly = Milk + possibility of Butterfly Egg
Chicken = Grain + possibility of Chicken Egg