Beef is favorite alm dish to offer to Thai monks. Freshly prepared before breakfast time, it's generosity and good merit.
However actually, I've never seen Thai Theravada nuns before. I've seen female lay devotees in white robes occasionally but not actually an ordained and shaven nun in orange robe. Nuns are more common in Mahayana.
I would not call the chicken stupid, it has no choice when it was born into this world. Even if it is not killed on the spot, it is still indirectly link to the nun on its death, although in this case the link is relatively weak.