题目

       Have you ever wondered why birds sing? Maybe you thought that they were just happy .After all ,you probably sing or whistle when you are happy.       Some scientists believe that birds do sing some of the time just because they are happy .However; they sing most of the time for a very different reason .Their singing is actually a warning to other birds to stay out of their territory.       Do you know what a “territory” is ?A territory is an area that an animal ,usually the male, claims（声称）as its own .Only he and his family are welcome there .No other famil