When birds chirp, people can feel a little peace. But some bird sounds are particularly pleasing to the human ear, and scientists have begun to understand why. For most of our evolutionary history, the sounds made by birds while they were busy with their own affairs may have been a reassuring signal that our environment was free of danger. A small new study has helped clarify why some birds' songs are more pleasing to humans than others. According to researchers at the University of Tübingen in Germany, humans tend to prefer gentle sounds with a narrow frequency range. They also...