Why are observers in temperate climates so fascinated by bird migration? Because birds make us feel like chumps as they head for a sensible climate and leave us to deal with the cold. But birds don't have maps -- so how do they know where they're going? And how do they get the energy to get there?
How do birds find their way? Simple. Through a combination of
- Sighting (they don't call it a "bird's eye view" for nothing) features like rivers, coastlines, and mountain ranges.
- Monitoring Earth's magnetic field, apparently with their visual system and with tiny grains of a mineral called magnetite in their heads
- Observing the stars
- Using the sun for guidance
- And probably following their neighbors (many birds migrate in large flocks)
Does it still sound simple? I didn't think so. For example, birds which use magnetic navigation must deal with a small problem -- magnetic north is 1,600 kilometers from the north pole. That means migrants leaving northern Alaska and following magnetic south would be travelling due west!
Second, star navigation changes as new constellations appear on the horizon as the birds travel north or south.
Although birds have apparently been overcoming these problems for millions of years, only in the past year have scientists figured out how at least one species of bird does it (see Constant Compass Calibration). It seems that the birds recalibrate their magnetic compasses against their star navigation during their rest stops along the migration route. And if they don't have enough time at the rests, they get lost.
Once the birds know where they're going, they also need a way to get there, and that brings us to the realm of fuel efficiency -- of miles per rodent, or kilometers per thistle seed. Depending on their size, route, and laziness (just kidding!), birds use one of these flight strategies:
Slow and Steady Wins the Race: The most basic technique is to keep flapping your wings until you land. That's the technique used by the Canada goose and many other migrants.
Soaring. Smarter (still kidding) birds have figured out how to ride "thermals" -- updrafts of air caused by solar heating, to take a free ride high into the sky. These birds, including Swainson's hawk, turkey vultures, and many others, only travel during the day, and only over land (preferably flat land). Those restrictions can lead to astonishing concentrations of migrating birds.
Flapping and gliding. These birds flap their wings for a few beats, then glide for a while. After they lose some altitude and/or speed, they flap some more.
Bounding. This is a combination of flapping with a closed-wing glide. It's used by birds whose wings would produce too much drag. Although their aerodynamic bodies do create some lift, the birds tend to lose altitude and the flight pattern is up-and-down, like the flap-and-glide pattern.
Rapid Transit: the ins and outs of bird migration
Let's face it--when it comes to dealing with winter, most birds seem an awful lot smarter than humans. Instead of griping about the weather, they simply head for a warmer climate. Let's look at a few facts on bird migration:
What's the record for the longest migration on the planet?
The arctic tern flies a phenomenal round trip that can be as long as 20,000 miles per year, from the Arctic to the Antarctic and back. Other sea birds also make astounding journeys: the long-tailed jaeger flies 5,000 to 9,000 miles in each direction.
The sandhill and whooping cranes are both capable of migrating as far as 2.500 miles per year, and the barn swallow more than 6,000 miles.
For the last word on bird migration, see the Atlas of Bird Migration.
Why do some birds go north for the summer?
Because there's more to eat. The 24-hour days near the Arctic Circle produces a fantastic flowering of life. This brief, but abundant, source of food attracts many birds (and mammals such as the caribou) to the Arctic for breeding purposes.
What influences migration patterns over the long term?
Changes in climate (particularly ice ages), and shifts in the positions of islands and continents as a result of tectonic drift.
How do they keep going?
Some birds store a special, high-energy fat before the trip. Soaring raptors, for example, may not eat for several weeks as they migrate. Other species eat along their migration routes.
How high can they fly?
Higher than Mt. Everest. Bar-headed geese have been recorded flying across 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.
(Note: birds don't fly this high just to get in the Guinness book of records, but rather to reach their destinations efficiently. From radar studies, scientists know that birds can change altitudes to find the best wind conditions. To fight a headwind, most birds stay low, where ridges, trees and buildings slow the wind. To ride a tailwind, they get up high where the wind is as fast as possible.)