Ready to Fly

For birders, spring migration is something to look forward to and be celebrated, but how do birds know when it's time to begin their travels?

- EMILY HANNEMANN

Flying north might seem daunting, but birds do it for good reasons.

Species that journey to the tropics for winter must compete with year-round tropical residents for food. When spring arrives in North America, resources are plentiful. There's food to eat, grasses and twigs to build nests, and warmth to enjoy. Flying north has its advantages—but according to birding experts Kenn and Kimberly Kaufman, it's still unclear how birds make the decision to go.

For a long time, they say, the answer was that birds responded to changes in the length of daylight. That worked to explain birds moving north from the southern United States as days grow longer in spring, but it doesn't explain why birds would leave grounds close to the Equator. Kenn and Kimberly believe birds could have an “internal migration calendar,” but science hasn't fully explained how that would work ... and for their part, birds seem content to keep their secrets.

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