What at one time might have been a short jaunt to and from a seasonal feeding ground might over time have become exaggerated as climates or resources changed.Ī key force in this evolution may have been the earth’s many cycles of environmental change, including, over the last 2.5 million years, more than 20 glacial cycles leading up to the most recent melting and retreat of vast ice sheets about 15,000 years ago. Individuals whose movements gave them better chances to survive and reproduce passed that migratory behavior along to their offspring.
One leading theory holds that birds started by making small annual movements as they searched for an edge in feeding or nesting opportunities. How migration might have evolved, via gradual steps, is a tougher question. Migrating birds are just following that abundance. After all, seasonal change is one of the most dependable features of our planet, providing predictable resources such as spring leaf-out, monsoon rains, insect hatches, and fruiting seasons. And hundreds of other species ranging from hawks to hummingbirds routinely perform their own epic migrations.Įxtreme as it may seem to a terrestrial primate like us, migration is for many birds a common-sense reaction to the availability of resources. Bar-tailed godwits can fly from Alaska to New Zealand in a single nonstop flight that lasts eight days. A four-ounce Arctic tern can fly to Antarctica and back each year during a lifetime that spans 30 years. To an earthbound species like humans, bird migration is nothing short of extreme.
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