If your mother wants to tell you something, she uses words. Birds can not talk as we do. But some birds can make sounds to warn their young of danger. They have their own way to make the young birds know or do certain things. The jackdaw is a kind of blackbird that lives in Europe. Jackdaws live together in groups. Young jackdaws do not know their enemies. When the older jackdaw sees a dog, it makes a loud rattling sound. The younger birds know this sound means an enemy is nearby. The sound warns them to know their enemies. If a young jackdaw is in a dangerous place, a jackdaw parent flies over him from behind. The parent bird flies low over the younger bird's back. The parent's tail feathers move quickly from side to side. It is trying to say, "Follow me." At the same time, the parent calls out, "Key-aw, Key-aw." The parent means, "Fly home with me." Young jackdaws do not have to learn what certain sounds mean. They know the meanings of these sounds from the time they are _ . Which of the following shows the main idea of the passage?
Birds cannot talk as we do. Some birds can make sounds to warn their young of danger. Birds have their own way to make the young ones know or do certain things. Young jackdaws do not have to learn what certain sounds mean.