For the Birds Radio Program: Migration Strategies
Different strokes for different folks, and different migration strategies for different birds.
Every year, millions of birds travel back and forth between Minnesota and Latin America. Fall migration is a long, arduous journey to make, during the height of hurricane season, and most of the birds making the trip were just hatched a couple of months before. Some birds migrate as a family, so young can learn from their parents their migration route, feeding strategies, and how to cope in strange new environments. This strategy is common with large birds such as geese, swans, and cranes, but also in some small birds such as swallows. When bird rehabilitators care for baby swallows, they have to go to great lengths to ensure that they introduce the fledglings to adult swallows that will “take them under their wing,” so to speak. Many of these young birds stay with their parents during at least the start of winter, but as days lengthen and parents get focused on raising a new family, they leave them behind. This is why the Whooping Cranes following the Ultralight airplane from Wisconsin to Florida need attention throughout migration and winter, but then return to Wisconsin entirely on their own in spring and, barring injury, never need assistance from their surrogate parents again.
Some adult birds head out well before their young. It seems as if loons and hummingbirds must be rather cold parents the way they abandon their babies, usually in late July or early August. But these birds depend on a supply of food that dwindles by summer’s end. If loon adults remained with their young, it’s possible they’d deplete the nest lake of fish before their babies’ flight feathers were long enough to support flight, and since loons aren’t adept at walking to reach other lakes, the babies would starve. If adult hummingbirds remained with their young through late summer when many nectar-bearing flowers are drying up, they could all go hungry. But because the adults light out for the territory early, the young have a higher chance of survival feeding on the remaining flowers as they put on enough fat to sustain their own flight south. This means, of course, that loon and hummingbird babies must have the timing, route, and survival strategies pre-programmed in their brains, in the form of instincts.
Contrary to what seems intuitively true, most birds don’t migrate as a response to dwindling food supplies—they migrate when food is most available to fuel their travels. In August, insects and fruits are abundant right when the birds that depend on them start moving. Nighthawk flights usually begin in mid-August, when the sky is abuzz with “aeroplankton.” Entomologists have collected masses of insects at surprising heights from small airplanes, We see our most spectacular nighthawk flights in late afternoon and dusk. Nighthawk migration isn’t well understood, but the birds seem to go higher and higher as darkness falls, so are likely migrating all night long, feeding as they wend their way toward South America. Swallows, feeding on that same insect food, fly by day, lacking the nocturnal vision of nighthawks. In August, we can find large collections of swallows on powerlines, but by September, even though their food is still plentiful, the bulk of their movement through Minnesota is already over.
Robins, orioles, and waxwings feed on fruits as well as insects. They start moving as soon as the babies become capable of long flights, and have already started moving. They’re drawn to cherry trees, berry-laden shrubs, and other rich sources of food, so fly fairly low in daytime, watching for convenient spots to drop down and feed. Orioles, with a much longer journey than waxwings or robins, make long flights at night as well. Most small songbirds make their entire journey under cloak of darkness, when temperatures are cooler, winds are lighter, and hawks are asleep. Celestial navigation aids their flight—from the time they are nestlings, birds notice the star patterns and how all the stars seem to rotate around the one fixed star, and set their first migratory course flying away from Polaris. During the day, they pig out on summer’s abundant insects to maintain enough body fat to sustain the journey.
Hummingbirds migrate by day, dropping down whenever they spot a flower or feeder, but once they reach the Texas coast, they spend several days fattening up before striking out over the Gulf of Mexico. It takes 18–26 hours to make that flight, with no place to land for resting or feeding, before they reach the Yucatan Peninsula, a feat all the more impressive since they make this journey during hurricane season.
Hawks migrate by day, and can count on the richest supply of all, migrating birds, to sustain them. We take a different form of sustenance from migratory birds, finding, as Rachel Carson put it, “something infinitely healing in the repeated refrains of nature.”