Final answer:
About 20% of bird species do migrate long distances annually, a fact that is supported by various examples of bird and animal migrations, which tend to be due to changes in seasonal resource availability.
Step-by-step explanation:
The statement that about 20% of bird species migrate long distances each year is true. Bird migration is a well-documented form of seasonal movement reflecting changes in resource availability. This behavior helps them to access warmer climates with sufficient food during winter, as seen in species such as the geese, which migrate in V-shaped formations, and the emperor penguins documented in the film March of the Penguins undertaking a 62-mile journey in Antarctica. Additionally, some species exhibit what is known as facultative migration, where they may choose to migrate or not depending on environmental conditions. Conversely, Swainson's hawks, mentioned in Figure 8.5.1, illustrate the incredible distances some birds travel, and migrations are not limited to birds, as many other animal groups, including mammals like wildebeests and whales, also engage in long-distance migrations.