It depends on how your chickens are kept while outside. If they free range, there should be lots of cover for them to run under when they see/sense a hawk is around. Chickens learn to worry about shadows up above them and one usually sounds an alarm, unfortunately it takes a little while for them to learn sometimes. I'm sorry you lost some of your birds.
If they are in a run, it's easier that's for sure. you can criss-cross the top with monofilament line (heavy duty fishing line) and hang CDs around on it. Hawks can't manuver through the line well, and they don't like the spinny reflective light off of the CDs.
If they free range, you can still hang CDs around the yard. I started searching for some of the pictures here, maybe someone else will post the links too, there's a BUNCH of examples of how people have done this.
None of this is perfect, hawks are a danger and you're right, they are totally protected by federal law so it's a good thing you stopped your brother. All migratory birds are and that includes all hawks and eagles even though they don't always migrate. The good thing is, this red-tail may be on his way to another winter grounds and he will be on his way soon.
I'll search for hawk deterrents more too.
