if they havent layed their eggs or hatched chicks yet (i have no idea what the breeding season for hawks is) then you could probably stress them out so much that they leave and try to find a new nest. It's borderline legal, and kind of humane (they aren't physicaly getting harmed) but thats what i'd do if i was in that situation. Firing loud blanks towards them, flashing bright lights at night, hitting their nest with paintballs etc. would probably get them to leave. Hanging up cd's around the bird pen may work for a little while, but isn't a permanent solution. I had a sharp shinned hawk (could've been a coopers) rip down strings of cd's to get to my doves, the cd's had only been up for a week. Over the years I'v lost probably close to 15 birds to hawks, mostly doves, all of which were in their secure pens, never lost a single one to any other kind of predator, i kind of have a vendetta against all hawks now. good luck.