From Susan Krech:

Re: What? Where? When? Some animals may know (Aug. 12): Many years ago, I was training as a hill shepherd on the English/Scottish Borders. The flock we were working with was kept on the Otterburn Firing Range. Not only the Royal Air Force, but several European air forces used this area for training in low-level flying. As you can imagine, this made life a little stressful for the sheep, as well as for those of us not used to seeing jet fighters flying level with us as we worked on rebuilding dry stone walls! It was amazing to us at the time, but the sheep had learned to recognize when flying was done for the day, and sure enough, at quitting time, they all came out of hiding and spread out over the hills. When flying was due to begin the next day, they all quietly vanished into safer areas. Of course, there is always the flocking instinct to keep in mind, but those sheep definitely knew what time of day it was.