ames, chickens can be very aggressive (social hierarchies are referred to as “pecking orders” because that’s how chickens establish a social order in the flock). I wouldn’t be surprised if a chicken pecked to death would also be eaten.
The meat byproducts that are fed to factory chickens probably include poultry scraps too.
But it’s probably true that kosher birds do not eat other birds as a regular part of their natural diet.