Hummingbirds are among the most fascinating creatures in the avian world. The male Anna hummingbird, the most common on the U.S. west coast, makes a high-pitched chirp, or screech, when it dives to guard its nest. But it doesn't use its voice.  How it makes that sound has been a mystery until now. Two young researchers at the University of California have solved it, using a clarinet, the bird's tail feathers and a wind tunnel.  VOA's Paul Sisco explains.