TOEFL Lecture: How Birds Navigate

Professor: OK, last time we looked at why birds migrate. Today, the harder question: how do they find their way? There are three main ideas. First, the sun. Many birds use the sun's position, and they correct for the time of day, which is pretty remarkable.

Professor: Second, the stars. Experiments in planetariums showed that some songbirds orient by the stars around the North Star.

Student: But what about cloudy nights? They still migrate then, right?

Professor: Good question, and that's exactly where the third idea comes in: the Earth's magnetic field. Birds seem to sense it, possibly through proteins in their eyes. Now, I'll be honest, the details are still debated. But I find the evidence convincing. Birds released under heavy cloud still head the right way.

Professor: So for the exam, remember it's not one system. Birds combine all three, and switch depending on conditions.