Why Do Animals Migrate? by Bobbie Kalman is my curriculum tie-in this week. I'd recommend this to grades 3-5. It is a chapter book; 32 pages in total.
It starts with definitions. Example: "All animals have areas where they live. These areas are called habitats." The book gives the reason for migration: because when seasons change it becomes too difficult to stay in their habitats. So they move to a new habitat for a certain period of time. They migrate.
Teachers, here are two questions to ask of your classes.
Q. Why do Canada geese fly in a V-formation?
A. Flying in a V-formation keeps them from getting too tired. p.10
Q. Which animal migrates the farthest?
A. The arctic tern. p.12