The Many Styles of Bird Migration Are More Varied Than You Think
BY LAUREN LEFFER | ILLUSTRATIONS BY LIZ WAHID | SPRING 2022
At least 40 percent of bird species migrate in some form. But it’s simplistic to think of these trips as only annual flights south in fall and north in spring. Migration spans a spectrum from a weeks long pilgrimage to a short meander. A bird might depart on the same day every year to a specific destination or follow irregular cycles. Most birds fly, but some walk, swim, or even stomach slide. Understanding migration means embracing a range of unique, astounding, and downright weird strategies.
White-winged Crossbills are nomadic, known to wander North America’s boreal forests in big flocks while seeking out conifer seeds. These birds travel great distances to access abundant food, from Alaska to Maine and points in between. The timing and path of their movement varies year to year—and if good cone crops persist, they’ll stay put.
Northern Bobwhites in the Smoky Mountains are altitudinal migrants. Instead of moving to different latitudes with the seasons, they move to lower elevations for food in winter and higher elevations to breed in spring. Theirs is one of the very few altitudinal journeys in the eastern United States.
Eared Grebes embark on molt migrations. On annual travels, they make long stops at hypersaline lakes in Utah and California to shed and regrow feathers. While the waterbirds are flightless for months, the unique habitat offers a feast of brine flies and shrimp and few predators. Their breeding sites can shift yearly, but their molting areas stay the same.
Not all American Robins migrate. Some stay put all year, while others fly thousands of miles—making the species a partial migrant. The decision to stay or go depends on many factors. If berries are scarce in winter, or if weather gets too harsh, birds may seek better conditions elsewhere. Climate change could be making year-round residents more common.
Spruce Grouses opt to migrate on foot, although they can fly short distances. Their slow, seasonal movements are small compared to other birds: less than 7 miles between breeding and wintering sites (and usually under a mile). Females are up to four times more likely to migrate but it’s unknown why.
When Scripps’s Murrelets leave breeding islands, newly hatched young can’t yet fly. Families migrate together to non-breeding territory along North America's Pacific Coast by swimming. Known to nest in only about 10 places, these rare seabirds are of conservation concern.
Some Snowy Owls migrate yearly, while others stay near breeding grounds. At irregular intervals, however, these birds are irruptive, moving en masse to a new location. In these years, snowies are a common winter sight in the United States and may fly as far as Florida. Why owls irrupt is somewhat a mystery: Theories include prey and weather fluctuations.
Bar-tailed Godwits undertake the longest nonstop migration known. Individuals have been recorded traveling more than 8,100 miles without food or rest and regularly flying more than 7,000 miles one-way in only nine days. To achieve this, their kidneys, stomachs, and livers rapidly change in size. The organs expand while a godwit fuels up to facilitate quick digestion and larger energy stores; they shrink before takeoff to make room for fat and flight muscles. One study found the birds’ fat composition went from 17 to more than 50 percent just before migration.
In March Emperor Penguins travel up to 100 miles by either walking or tobogganing on their bellies to reach inland breeding colonies in Antarctica. There, amid the planet’s harshest conditions, they lay their eggs.
Ruby-throated Hummingbirds beat their wings more than 50 times a second. The frenetic flapping makes their migration one of the most energy-intensive. In preparation for traveling from the Gulf Coast to Central America, birds store fat and double their weight. Although unconfirmed, scientists think some birds may undertake a risky nonstop route of more than 600 miles across the Gulf of Mexico.
Dusky Grouses head upslope in winter from their open breeding areas toward dense conifer forests—going in the opposite seasonal direction compared to most altitudinal migrants.
Richard’s Pipits, Eurasian songbirds, are one of the first species known to be shifting migration routes from a north-south to east-west direction, likely in response to climate change.
Common Swifts, with the help of the wind, are among the fastest known migrants and can cover more than 500 miles of ground per day as they travel between Africa and Eurasia.
Ospreys, by contrast, take their time, often stopping or diverting course to fish. One tracked male spent 20 days mid-migration in Cuba before continuing on to Brazil.
Illustrations: Liz Wahid. Web adaptation: Alex Tomlinson/Audubon.