The
ASPCA lists yarrow as "Toxic to Cats, Toxic to Dogs, Toxic to Horses"
Notice that the "Clinical Signs: vomiting, diarrhea, depression, anorexia, hypersalivation." My guess is that the animal is quickly sorry it ate the stuff because it causes digestive distress. It looks like something a puppy would get in trouble with because it chewed on the plant.
Yarrow is also a weed that could be harvested with a hay crop. Still, a good source for farmers,
Canadian Poisonous Plants, doesn't bother to list it.
If your plant in the yard is one of those big, golden Achillea filipendulina - I have a hard time imagining anything "wanting" to eat one of those strong-smelling things!
If it was me, I just wouldn't throw the leaves in a pen of bored chickens. You probably wouldn't be doing them any favors. Other than that, I think they would have sense enough not to eat it if there are other plants available to them.
. . . just my 2¢.
Steve