Why Do Birds Migrate?
By Linda Ewing
For many birders, spring and fall migration are the best times of the year. In the spring, migration provides a welcome shot of color; in the fall, it’s an opportunity to hone identification skills on birds in drabber, non-breeding plumage. In both seasons, it’s full of surprises and variety and wonder.
But why do birds migrate? Given the hazards, from building collisions to disease to predators to just plain exhaustion, how can long-distance migration possibly make evolutionary sense? And how on earth do migrating birds know when to leave and where to go?
This article looks at what we know about bird migration, and at what researchers are still trying to understand. It also suggests actions bird lovers can take to make migration safer.
Table of Contents
- What drives bird migration?
- How do migration patterns vary?
- How do birds know when and where to migrate?
- Migration champions: feats of endurance
- Why migratory birds are at risk
- Supporting migratory birds
What drives bird migration?
Birds, like other animals, are driven by survival – their own and that of their offspring. At first glance, this makes migration even more mysterious. Why would birds expend tremendous amounts of energy to fly across continents and oceans, instead of finding a spot with plenty of food and settling there?
To ask that question is to answer it. The portion of the globe in which a wide variety of fruits, seeds, insects, crustaceans and other foods are plentiful year-round is limited. Regions where that is the case are the exceptions that prove the rule. Not only are they blessed with incredible avian diversity, including seasonal migrants from less-favored zones, they also tend to have particularly large numbers of year-round residents. In regions with harsher climates, winter food is limited, and the barren landscape provides less protection from predators and the elements. Migration allows birds to breed in areas they could not survive year-round, spreading avian life across a wide variety of climates and habitats.
How do migration patterns vary?
The classic migration story has birds hard-wired to follow the same flyways they have used for millennia, heading north to their breeding grounds in the spring and back south to their wintering grounds in the fall. That’s not wrong, but it’s too simple.
First, not all birds that breed in northern latitudes migrate. Year-round residents are most likely to be seed eaters, like Northern Cardinals, or omnivores, like Northern Mockingbirds. A few non-migratory species have evolved alternative survival strategies. One of the most unique is that of the Common Poorwill, which enters a hibernation-like state called torpor for weeks at a time to conserve energy during cold spells.
Some species have both migratory and resident populations that intermingle across broad swaths of their range. To further complicate matters, climate change and human settlement are scrambling migration patterns. American Robins were once considered a harbinger of spring in the northern U.S.; today, significant numbers remain on their breeding grounds through the winter.
Still other species are irregular migrants, sometimes remaining on their breeding grounds and sometimes wandering long distances in unpredictable directions, a phenomenon known as “irruption.” Bohemian Waxwings, Snowy Owls and many northern finches are good examples of irruptive migrants, driven by food supplies rather than seasonal cues and hard-wired maps.
Even among birds that do migrate seasonally, the trajectory isn’t necessarily north-south. Some species migrate by altitude rather than latitude. American Dippers are a good example, spending the summer in high mountain streams and moving into surrounding valleys in the winter.
How do birds know when and where to migrate?
Early laboratory experiments, combined with observation of birds in the wild, made it clear that migration is not primarily a learned behavior, but something innate. You could say it’s written in avian DNA. And in fact, advances in gene sequencing are allowing scientists to zero in on specific areas of the genome associated with migration. For example, researchers have linked the winter destinations of Golden- and Blue-winged warblers to a single gene.
These genetic instructions interact with environmental cues in complex ways that we’re still unraveling. Changes in the length of the day signal that it’s time to move, though even birds deprived of natural light experience a kind of migratory restlessness. Once on their way, birds use a variety of navigation techniques to stay on their genetically programmed course. Diurnal migrants can follow visual landmarks, but most migratory birds fly at night or over featureless expanses of water, guided by the position of the stars and the rising/setting sun or even, in the case of some seabirds, by their sense of smell.
There’s even evidence that migratory birds have a sense we lack, one that allows them to perceive the earth’s magnetic field through the interaction of blue light photons with special proteins in their eyes.
In addition to their internal programming, birds have a surprising capacity for learning and memory. Young birds seem to get better at migration as they mature, flying more strongly and efficiently and adjusting their routes to take better advantage of wind currents. As for memory, it’s not uncommon to see individual birds return year after year not just to the same general area, but to a specific site.
Migration champions: feats of endurance
The developing science leaves ample room for wonder at the feats of endurance these small, feathered creatures accomplish twice a year.
Consider the Arctic Tern, the undisputed migration champion. Weighing just 4 ounces – the same as a stick of butter – these birds spend roughly four months out of the year in active migration, traveling between the extremes of the northern and southern hemispheres. Some individuals have been tracked covering more than 55,000 miles round trip. This is not a once-in-a-lifetime achievement; Arctic Terns can live for upward of 30 years, repeating their odyssey year after year.
Red Knots are another long-distance migrant, traveling some 18,000 miles annually. To prepare, Red Knots build fat stores that can double their weight. The bulking up is selective, though. Body parts that aren’t essential to flight, like leg muscles and digestive organs, shrink. Meanwhile, the knots’ pectoral muscles and heart grow larger. Once the birds reach their destination, these physiological changes reverse.
A Ruby-throated Hummingbird could nestle comfortably in the palm of your hand and weighs about the same as a penny. Most of the year, these birds must sip nectar constantly to sustain their high metabolism. During migration, fueled by a pre-flight feeding binge and aided by the wind, they cross the Gulf of Mexico in a non-stop, 20-hour flight.
Why migratory birds are at risk
Birds’ migratory journeys have never been easy, but human beings are making them harder.
Habitat and food source destruction at key stopovers is a major threat to long-distance migrants. Examples are depressingly plentiful, but a few stand out. In the desert west, diversion of water from saline lakes – most notably the Great Salt Lake – threatens the millions of migrating birds that rest and refuel there. On the mid-Atlantic seaboard, where horseshoe crab eggs fuel the epic migration of Red Knots, over-harvesting of the crabs means more birds set off for the Arctic underweight.
The migratory flyways that birds are hard-wired to follow take them over some of the most densely populated regions of the country. Big cities mean bright lights, which disorient nocturnal migrants. Add lots of windows to the mix, and the result is an epidemic of bird-building collisions. An estimated billion birds die each year from building strikes in the U.S. alone.
There’s also evidence that rapid climate change is throwing ecological relationships out of sync. A warming Arctic means insects emerge earlier in the spring. Birds, meanwhile, continue to follow their internal migration calendar, arriving on their breeding grounds at the same time they always have. When their chicks hatch, peak insect availability may have already passed – and less food translates into lower survival rates.
Supporting migratory birds
Migratory birds need human advocates. In Chicago, New York and other cities, bird lovers have organized to make migration safer by getting the managers of large buildings to turn their lights out during peak migration nights. Other initiatives have focused on mandating bird-safe glass for new construction and retrofitting existing buildings with window decals.
On a personal level, individuals can support sanctuaries that protect key stopover habitats. You can even create your own mini-sanctuary by incorporating native plants into your yard, adding water sources and minimizing your use of pesticides and chemical fertilizers. Our backyard birds collection celebrates many of the species that benefit from these small acts.
It’s up to us to ensure that future generations can also marvel at the spectacle and mystery of bird migration.