You're lucky that this hasn't happened before. I have hawks in the yard pretty much every day. Hawks are protected by federal law so you can't legally do anything that would harm the hawk.
I hope this hawk is just passing through, but he HAS noticed your birds and might hang around until he's figured out that he can't get them. Here are some links with tips for discouraging hawks:
http://www.rspb.org.uk/makeahomeforwild ... hawks.aspx Look down near the bottom under 'Deterrents'.
http://www.backyardchickens.com/t/73540 ... eter-hawkshttp://nature.gardenweb.com/forums/load ... 36.html?93 This one has an annoying amount of chitchat. The most helpful post says "You may be able to discourage the hawk by foiling the roost. Run a wire with some dangling things to discourage the hawk from setting on the fence. The wire would need to be suspended above the top fence rail but floppy enough to make it unstable. A hardware store should have what you would need to do this. We use this sort of set up to keep large birds from rooting on boats (for obvious reasons). But, if there are trees nearby, the hawk will just change position. You could also go out with the dogs at first to scare it off."
In your case, hanging some dangly stuff off the aviary roof might help keep the hawk away from the aviary wire. At the same time, try to create a sheltered area inside the aviary where the birds can hide if they feel threatened. That's partly what the cloth-shaded nook inside my aviary is for.
http://home.comcast.net/~ch_zonie/crafts-carpentry.html Look at the last picture in the second row.