It's one of nature's greatest mysteries: how do birds know the right time to migrate? Here's why migration happens — and how the journey occurs.
Why Do Birds Migrate North in Spring?
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. For birders, spring migration is something to look forward to and be celebrated, but how do birds know when to migrate? 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. “Near the Equator, the length of daylight hardly varies,” Kenn and Kimberly say. “Birds spending the winter there have to have other clues [to fly north].” They 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.
Timing Shifts: A Question of Climate Change
As weather patterns have shifted in certain regions, some local migration patterns have shifted too. Kenn and Kimberly mention the great-tailed grackle, which has become a year-round resident in some southwestern areas where it used to visit only in summer. In general, though, a warmer spring won’t lead to an earlier migration. Long-distance migrants fly based on their instinct. These instincts may change over time, but that change is a slow process over many generations.
Birds that migrate shorter distances, however, might be affected differently. “Some short-distance migrants, especially those that spend the winter in the southern United States, may start to move north earlier than usual if the temperatures stay consistently mild for a lengthy period in late winter and early spring,” Kenn and Kimberly say.
They add that milder winters could potentially affect birds that migrate north from the U.S. as spring arrives, but crossbills and redpolls, for example, migrate based on food rather than weather.
Risks and Benefits to Migration for Birds
Migration doesn’t come without its challenges. Birds face risks from storms, habitat loss and urban light pollution. All of these complicate the journey, making it a balance of cost and reward. “If a bird heads north too early, there’s a strong risk of running into lingering winter weather that may prove to be fatal,” Kenn and Kimberly say. “The ideal timing is to reach the breeding grounds just after the worst weather is over, and just before most other birds of the same species have arrived, so good territories in prime habitat are still available.”
Flying Solo: Most Birds Migrate Alone
Honking geese flying in a V formation is a quintessential image of migration, but it’s not the best representation of how most birds go north. “Most kinds of migratory birds are somewhat independent: They migrate as individuals,” Kenn and Kimberly say. “If they join a flock, it’s just for convenience to be with other members of their species that are going the same direction.”
That said, there are some exceptions to the rule. Many migratory birds don’t have a strong social structure that would guide them to migrate as a group, but Kenn and Kimberly list cranes and geese as two types that go north in flocks. In those cases, migratory decisions are often made by the elders. “The older and more experienced birds will decide when and where to migrate, and the younger birds will simply follow along,” Kenn and Kimberly say. Even in those groups, there can be surprises. “Some individuals may just set off on their own,” they say.
Learn even more about bird migration with these stunning migration secrets.
Attract More Migrating Birds
Here’s how to keep surprise visitors happy and healthy
It’s easy to give your favorite migrants a helping hand. Kenn and Kimberly recommend setting out high-quality foods such as black oil sunflower seeds, white proso millet, Nyjer and suet. In the long term, they suggest growing native plants to provide shelter for migrating birds in any season.
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