It turns out that arsenic has routinely been fed to poultry (and sometimes hogs) because it reduces infections and makes flesh an appetizing shade of pink. There’s no evidence that such low levels of arsenic harm either chickens or the people eating them, but still…
The same study also found that one-third of feather-meal samples contained an antihistamine that is the active ingredient of Benadryl. The great majority of feather meal contained acetaminophen, the active ingredient in Tylenol. And feather-meal samples from China contained an antidepressant that is the active ingredient in Prozac.
Poultry-growing literature has recommended Benadryl to reduce anxiety among chickens, apparently because stressed chickens have tougher meat and grow more slowly. Tylenol and Prozac presumably serve the same purpose.
Researchers found that most feather-meal samples contained caffeine. It turns out that chickens are sometimes fed coffee pulp and green tea powder to keep them awake so that they can spend more time eating. (Is that why they need the Benadryl, to calm them down?)