2026年1月托福机经套卷(第8套) - Read an Academic Passage Q3

Bird Migration Every year, millions of birds travel huge distances to wintering grounds, and then back to breeding grounds when warmer weather returns there. Bird migration evolved in response to climatic changes. During the Ice Age, when average temperatures were frigid, migrating birds had a survival advantage, and the behavior became widespread. Migration is also linked to resource availability. Arctic terns, which undertake the longest migrations of any animal by flying from their summer breeding grounds in Earth's north polar (Arctic) regions to their wintering grounds in Earth's south polar regions, feed primarily on small fish and other small marine animals. This prey is most abundant when increased sunlight results in the increased availability of algae, the microscopic marine plantlike organisms that are food for the tiny marine animals known as zooplankton, which are in turn consumed by terns' prey. Not all birds migrate. Rock ptarmigans also live in Arctic and sub-Arctic regions. Instead of flying to warmer climes in winter, they shelter in snow burrows and reduce activity to conserve energy. They feed on plants like birch and willow trees, which are available in their habitat year-round. The divergence between migratory and sedentary species represents different adaptations to environmental pressures.