根据短文内容，用方框中所给单词的适当形式填空，每词仅用一次。

Most people think birds sing because they feel happy. It seems so But scientists find that they sing most of time a very different reason. Birds do not welcome others to their own homes except the family members. When a tries to get close to their homes, they got angry and kept loudly to frighten it away. So their singing is actually a warning to other birds to stay away from their homes. Next time, when you hear a bird singing, listen carefully and maybe you'll understand their special of saying NO.