POSTED 4 OCTOBER 2007
Long before butterfly migration was discovered, naturalists and scientists in temperate climates were fascinated by bird migration. A cynic might say they just felt jealous watching birds fly to a sensible climate, while they prepared to shovel snow and chop ice up north.
Birds may have motivation, but they don't have Mapquest. To reach a distant location, birds require orientation (a sense of direction, or a "compass") and navigation (the ability to find the way to a destination, which also requires some kind of internal "map").
Birds find their way by:
- Sighting features like rivers, coastlines, and mountain ranges (they don't call it a "bird's eye view" for nothing!),
- Monitoring Earth's magnetic field, apparently with tiny grains of the mineral magnetite in their heads,
- Observing the sun and the stars (especially polar stars, which are fixed above the poles),
- Smelling their surroundings (which is probably only helpful for short-distance travel), and
- Following the leader (many birds migrate in large flocks, although this does not explain how the leader finds its route...).
Devils lurk in the details. For example, birds that use magnetic orientation must deal with the fact that magnetic north is 1,600 kilometers away from the north pole. Migrants heading due magnetic south from northern Alaska would actually be traveling due west on the map! As the bird moves south, magnetic north starts to hew closer to true north, forcing the bird to continuously adjust its "compass" during migration.
Presumably, migratory birds have been overcoming this little snafu for millions of years, perhaps because they can integrate different navigational cues. One possible cue is polarized sunlight - light that has waves along a specific plane perpendicular to the light's forward direction. A recent study found that savannah sparrows "recalibrate" their compasses by reference to polarized light at dawn and dusk (see #1 in the bibliography). The authors suggested that many migrating birds may correct their bearings with this method.
The details are still a bit murky, but "Most scientists believe that birds posses redundant systems, and they use the 'best' system for the prevailing circumstances," says Stanley Temple, professor emeritus of wildlife ecology at University of Wisconsin-Madison. "They use the highly reliable celestial systems when the sky is clear, but switch to less reliable systems such as geomagnetism when the sky is overcast. Thus, they are very good at navigating when the primary clues are available but less accurate when they have to rely on 'back-up systems.'"
There's no question that questions remain about how animals find their way on their long journeys. According to a 2006 sum-up of migration science, "Yet after longstanding and intensive migration research, we are still far from a fundamental understanding of animal navigation, and the emerging picture is complex and intricate" (see #2 in the bibliography).
Once the birds know where they're going, they also need to get there, and that brings us to the question of "fuel" efficiency -- of miles per field mouse, or kilometers per thistle seed. Depending on their size, route, and laziness (just kidding!), birds may choose from three basic flight strategies:
- Flap. The fundamental technique -- beat-your-wings-until-you-arrive -- is used by the Canada goose, among many other migrants.
- Flap and glide. These birds flap their wings for a few beats, then glide for a bit. After they lose some altitude and/or speed, they flap to regain altitude and speed, producing an up-and-down flight pattern. A variant, called bounding, combines flapping with a closed-wing glide. Bounding helps where outstretched wings would produce a lot of drag (air resistance).
- Soar. The most energy-efficient birds have figured out to hitch a ride on thermals (solar-heated updrafts) high into the sky, then soar gradually down toward another thermal. Soaring birds like Swainson's hawks and turkey vultures travel only by day, and prefer flat terrain where thermals are likely. The preference for these conditions can produce astonishing concentrations of migratory birds, especially over the western shore of a large body of water. The largest known concentration of migratory raptors, involving millions of hawks and vultures, passes through the state of Veracruz, Mexico, twice a year.
Q: What's the record migration?
A: The sandhill and whooping cranes can migrate up to 2,500 miles per year, and the barn swallow more than 6,000 miles. The arctic tern commutes between the Arctic and the Antarctic, and easily taking the long-distance prize with an annual round trip of at least 20,000 miles. Another frequent-flying sea bird is the long-tailed jaeger, which flies 5,000 to 9,000 miles in each direction. Albatrosses perform long flights across the Pacific, shunning land except for breeding, and sleeping on the sea surface.
Tracks from Laysan (yellow) and black-footed (red) albatrosses criss-cross the North Pacific. New satellite technology is giving a better picture of these long-distance, open-water migrants. Courtesy: Courtesy Scott Shaffer and TOPP
Q:Why do about 520 of the 650 bird species that nest in the United States migrate south for the winter?
A:Because they would freeze in the dark. Because there's little to eat in the northern winter. And because their ancestors did it. (But don't take this honoring-the-ancestors bit to mean slavish imitation: conditions in the United States have changed dramatically since the height of the ice age 20,000 years ago, and birds did adapt to that -- at least, the surviving species did.)
Q: If the tropics and sub-tropics are so mellow in the winter, why do birds bother returning toward the north or south pole in the summer?
A: Because there's more to eat. The longer days near the Arctic Circle produce a brief but fantastic flush of life, making an abundance of food that attracts many birds (and mammals like the whale or caribou) northward to the Arctic for breeding.
Q: How do migrating birds keep going?
A: Some birds store a special, high-energy fat before the trip, often nearly doubling their body weight before departing. Some birds, like soaring raptors, may not eat for several weeks as they migrate, relying instead on stored body fat. Other species perform voracious "stop-'n-gobble" ceremonies along their migrations. The need to rest and refuel along migratory routes explains why it's so important to preserve natural habitat along those routes.
Q: How high can migrating birds fly?
A: At least as high as Mt. Everest. Bar-headed geese cross the Himalayas at 29,000 feet. Other species seen above 20,000 feet include the whooper swan, the bar-tailed godwit, and the mallard duck. (Birds don't fly this high just to impress the Guinness book of records, but to reach their destinations efficiently. Radar studies show that birds change altitudes to find the best wind. To fight a headwind, most birds stay low, where ridges, trees and buildings slow the wind. To ride a tailwind, they rise to find the speediest air.)
Q: Is migration genetic?
A: Since at least 1968, ornithologists have thought that genetics plays a role. If you cage a migratory leaf warbler during migration season, it will hop restlessly. Even more important than the genetic cues for departure is a "genetic 'map' that birds use for navigation," says Temple. "Without it all the orientation skills in the world would never get you where you want to go." However, because migration patterns seem to change rather quickly, and closely related species have different maps, genetics does not explain the whole phenomenon of migration.
A 1968 study found that a caged leaf warbler gets restless during migration season, indicating a genetic control over migration. Courtesy: US Fish & Wildlilfe Service
Q: So birds have some flexibility in their migration patterns?
A: Yes. "Evolution primes birds to respond appropriately to 'average' conditions, but they must have the flexibility to cope with the perverse vagaries of the earthly environment," says Temple. In cold springs, migratory birds delay their flights north. New research on the northern wheatear, which winters south of the Sahara, and summers in Eastern Canada, Greenland and Scandinavia, shows this flexibility. German researchers studied wheatears at an island stopover in the North Sea, and saw that the appetites of different subspecies depended on their summer destination. Birds preparing for a short hop to Norway ate significantly less than those preparing for a 1,200-kilometer, blue-water jaunt to Iceland. Study leader Franz Bairlein, of the Institute of Avian Research, likened the birds to robots, but added that that they had "a surprising amount of behavioral flexibility." The birds departed the stopover earlier when many predators were present, but delayed their departures during stormy weather. Bairlein concluded that such flexibility could be an asset as climate changes (see #3 in the bibliography).
The Northern wheatear (Oenanthe oenanthe) has the biggest breeding area of any migratory bird in the world, covering much of the Northern Hemisphere. These birds can adjust their feeding according to the length of their next flight, indicating that migratory patterns are not genetically fixed. Courtesy: US Fish & Wildlife Service
Q: What factors influence migration patterns?
A: Many, including population changes among the birds or their food, movements in the positions of islands and continents due to tectonic shift, and changes in climate (particularly ice ages and now global warming).
Could global warming migratory birds?