Gr 3–6—Whether traveling up and down a mountain (Dall sheep) or flying from one end of the Earth to the other (Arctic tern), animals migrate for a variety of reasons across land, sea, and air. As readers follow the journeys of many different critters, they learn about their physical characteristics, habitats, and life cycles. They also discover facts about species' threats from pollution, climate change, and habitat loss, as well as efforts to help them. Color photos, some full spread, illustrate every title. In each volume, a migration map aids in visualizing the animals' routes and a website hosted by the publisher connects readers with additional research links. Because each book stands alone, libraries with limited budgets might consider purchasing individual titles to supplement or update existing collections. Taken together the volumes provide a good overview.