Half of the bird species on Earth migrate (Elphick, p. 10), making a regular and predictable journey to a new range to breed in spring, and returning in the fall. Other animals migrate, of course: caribou, whales, monarch butterflies—but birds are the most obvious. Why do birds migrate?
Evolution of Bird Migration
A variety of factors have shaped bird migration over time: climate and climate change, geography and continental drift, and competition between species and populations for limited resources.
Ice ages are thought to have played a part, making large areas of Earth uninhabitable for long periods. Jonathan Elphick points out that climate fluctuations between ice ages have been common and sometimes rapid, putting intense pressure on species directly affected (p. 8). Birds that were able to move to more hospitable ranges and survive had an advantage over those …