题目内容：
根据下列材料请回答题：A male zebra finch chirps away to himself. Suddenly he notices a female bird, nearby: He realizes he has an audience and immediately changes his song. Can the female tell the difference in his performance? According to a new study, the female zebra finch knows. And she prefers the special trills he creates when he sings to her. A male zebra finch changes his song when singing to a female in ways that people can barely detect. But the female finch can tell the difference.
Scientists had noticed slight variations in the songs of male zebra finches based on whether they were singing alone or whether there was a female ( and potential mate) nearby. With an audience, the males sped up the pace of their songs and controlled the notes they used. For this Study, researchers Sarah C. Woolley and Allison Doupe at the University of California, San Francisco decided to focus attention on the listening females, which have not been well studied in the past.
In the study, Woolley and Doupe set up a long cage with a sound speaker at each end. One broadcast the sound of a male zebra finch singing to himself, like someone singing in the shower. The other speaker broadcast a male performing for a female audience, as if he was giving a concert.
Female birds were placed between the two speakers. Some of the birds had mates, others didn't. The females shifted around a bit, and then most of them hopped over to sit beside just one speaker. All the birds that made a clear choice liked songs meant for a female audience, even if they'd never met the male.
Mated females also had a chance to listen to two different performance songs, one from an unknown male, and one from their mate. They spent more time listening, to the concert version of their mates' songs. This suggests that after a while, females learn to recognize—and prefer-the songs of their mates.
Scientists then studied the brains of the females. They found certain areas of the brain perked up when the birds listened to the concert songs. These brain areas may be involved in recognizing and evaluating the songs, and storing the memories of them. This research deals with what's called directed communication, when the communicator, or sender, focuses the message for a specific audience. One example is the way morns speak to their babies. Mothers around the world use the same sort of high-pitched sing-song chatter, and the babies respond best to those sounds. Songbirds are one of the only other species known to learn their communication, in this case their songs.
Which of the following is true about birdsongs?
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