Free CBEST Reading Practice Test

Every autumn, billions of birds leave their breeding grounds and travel thousands of miles to reach warmer wintering habitats — a journey driven by instincts that scientists are still working to fully understand. Research has revealed that birds rely on a sophisticated combination of navigational tools. Many species possess an innate sensitivity to Earth's magnetic field, allowing them to orient themselves even on overcast nights when stars are hidden. When skies are clear, they read celestial cues — the position of the sun during the day and the pattern of stars at night. As birds gain experience, they also memorize landmarks such as coastlines, mountain ridges, and river corridors. Remarkably, young birds undertaking their first migration seem to carry a genetic map of the general route, refining it with each successive journey until the path becomes precise. This layered navigation system makes migratory birds among the most impressive travelers in the natural world. Which of the following best states the main idea of the passage?