Most of the animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership (伙伴关系) which is good for both of them. You may see some birds sitting on the backs of sheep. This is not because they want a ride, but because they find easy food in the parasites (寄生虫) on sheep. The sheep allow the lairds to do so because they move the cause of discomfort. So although they can live without each other, they do better together.
Sometimes an animal has a plant partner. The relationship develops until the two partners can not live without each other. This is so in the corals (珊瑚) of the sea. In their skins there are tiny plants which act as "dustmen", taking some of the waste products from the corals and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from receiving light so that they cannot live normally, the corals will die.

Some birds like to sit on a sheep because（）