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   as its own. Only he and his family are welcome there. No other families of the same species  are welcome. Your yard and house are your territory where only your family and friends are welcome. If a stranger enter your territory and threaten you, you may shout. Probably this is enough to frighten him away.    If so, you have actually scared the stranger away without having to fight him. A bird does the same thing. But he expects an outsider almost any time, especially at nesting season. So he is screaming all the time, whether he can see an outsider or not. This screaming is what we call a bird's song, and it is usually enough to keep an outsider away.    Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter the territory of theirs.    You can see that birds have a language of their own. Most of it has something to do with attracting mates  and setting up territories. Some scientists believe that most of the time bird's singing is actually  _  .
showing they are happy a way of warning showing they are angry a way of greeting