I saw it on the BBC documentary "The Life of Birds" by Sir David Attenborough. Excellent program. It showed a hawk that can find rodents by seeing their urine from the sky. Amazing.
I would love to see it in person, though. When I go to the local wildlife refuge in the winter, when all of the migratory birds are overwintering here, I can park my car and wait for the eagles to go over. They are HUGE, the shadow cast over me looks like what a pterydactyl (sp?) would have. Ducks and geese pop up from all over and scatter. They don't scatter like that for hawks or harriers, just for eagles.