Poultry are fed a rich variety of feed to enhance and speed their growth. They also receive injections for diseases & the prevention of them. They are also given growth hormones.
We were able to verify that the Mehadrin Poultry (now includes the Vineland, Skver brands among others) only feed their chickens vegetarian feed and the injections are done in the egg not later than 18 and a half days. We don't know what the others are doing at this time.