阅读短文，根据上下文内容选词填空。

 swan   warm ducklings  back  cold into  ugly  swim

    In spring, itis sunny and .Mother Duck has four . But she is surprised(惊讶的). Because three ducklings are small and yellow. The last oneis big and grey. It's . The ugly duckling is very sad. His three brothers can sit on their mother's .But he can't. He away. In winter, it is .The ugly duckling has no home. The ugly duckling grows and grows. It is spring again.The ugly duckling looks  the water. How beautiful! He is big and white now. He is a beautiful ! He is very happy.