[解析]
(1)B 细节理解题.根据文章第二段最后一句Their singing is actually a warning to other birds to stay out of their territory可知很多的鸟儿唱歌是警告别的鸟儿.故选B.
(2)A 词义推测题.从A territory is an area that an animal,usually the male,claims (声称) as its own.可知territory是指鸟儿认为是自己的领地的地方.故选A.
(3)D 细节理解题.从you have actually frightened 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 (筑巢) season.可知它在筑巢期叫的原因是帮助吓退入侵者.故选D.
(4)B 推理判断题.文章很多地方都拿人与鸟儿进行比较,如If so,you have actually frightened the stranger away without having to fight him.可知是与人类比较的,故选B.