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 that could not. Many species that could not move died out when glaciers covered their home ranges, or changing climate destroyed food supplies.
Within any habitat, animals compete for resources such as food, shelter, and homes. For birds, an abundance of food is vital for breeding. When a bird species expands its range, individuals at the periphery often have less competition for food and other resources, and therefore do better during the breeding season, successfully raising more young. When they move into an area where there is more seasonal variation in temperatures and food supplies, however, it may be necessary to return to the home range during the winter. While this is migratory behavior, it once again concentrates the population in a smaller area and causes increased competition—if some individuals go somewhere else instead, they may have a survival advantage similar to that enjoyed at the edge of the range. Scientists believe that this sequence of events lies behind the summer and winter ranges of many long distance migrants.
The changing geography of Earth has also shaped bird migration. Continental drift has altered migration routes over millions of years. Mountain ranges, large bodies of water, deserts and river systems have directed migrating birds in various ways, and continue to do so.
Advantages of Migration
Migration takes advantage of the wealth of food available in temperate and arctic regions of the world during certain times of year. These regions come to life in the warmer parts of the year, but cold temperatures and physical barriers of snow and ice cut off the food supply in winter. The ability to take advantage of the summer bounty and move elsewhere when temperatures fall favors the survival of migratory birds and reinforces their movements.