Lengthening days set off a cascade of events in migratory birds that culminates in the birth of a clutch of chicks
Every spring countless warblers, geese, hummingbirds, and more—one in five bird species, all told—leave their winter hideaway on a long trek that eventually leads to mating, nesting and hatching chicks. Knowing when to leave is a delicate calculation for these animals, however. They need to reach breeding sites in plenty of time to successfully reproduce. But if they arrive too soon, winter may not have quite loosened its grip, which puts the birds at risk of starvation.
Birds molt, for instance, shedding their worn feathers and growing new ones. Their hormones change, influencing their digestive patterns and leading them to eat more, put on weight and build up flight muscle. In some birds, once they’ve eaten enough, a digestive hormone called ghrelin begins to make them restless and nudges them to take flight. And the birds’ reproductive tracts begin to change in order to ensure that they will be fertile by the time they breed.
Each bird undergoes these seasonal changes a little bit differently. “They’re all sort of like little machines in terms of getting turned on, but there are individual decisions,” Ramenofsky says. Understanding migration is also complicated because the phenomenon comes in many forms. Some birds, such as Savannah Sparrows and American Woodcocks, migrate only short distances. Others, such as Cerulean Warblers and Red Knots, begin their prebreeding migratory journeys in the Southern Hemisphere, where days are getting shorter, not longer, and then cross the equator to reach North America.
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