A、To find more food. B、To produce their young. C、To keep themselves warm. D、To protect themselves better. B选项均以“To+原形动词”开头表明，本题可

Along the river banks of the Amazon and the Orinoco there lives a bird that swims before it can fly, flies like a fat chicken, eats green leaves, has the stomach of a cow and has claws on its wings when young. They build their homes about 4.6m above the river, an important feature for the safety of the young. It is called the hoatzin.
    In appearance, the birds of both sexes look very much alike with brown on the back and cream and red on the underside. The head is small, with a large set of feathers on the top, bright red eyes, and blue skin. Its nearest relatives are the common birds, cuckoos. Its most striking feature, though, is only found in the young.
    Baby hoatzins have a claw on the leading edge of each wing and another at the end of each wing tip. Using these four claws, together with the beak, they can climb about in the bushes, looking very much like what primitive birds must have done. When the young hoatzins have learned to fly, they lose their claws.
    During the drier months between December and March hoatzins fly about the forest in groups of 20 to 30 birds, but in April, when the rainy season begins, they collect together in smaller living units of two to seven birds for producing purposes.
Questions 22 to 25 are based on the passage you have just heard.
22. What is the passage mainly about?
23. Why are young hoatzins different from their parents?
24. What can we infer about primitive birds from the passage?
25. Why do hoatzins collect together in smaller groups when the rainy season comes?