Wake up early enough and probably you     1     （hear） a pleasant chorus of birds. This early singing can start as early as 4 a. m. and last for several hours. Birds do it     2     （attract） mates and warn other birds to avoid their place. One theory as to     3     they choose the early morning to communicate these important messages is that the low visibility makes it hard to do other bird activities, like searching for food. Keeping their activity level low, they choose to sing instead.