Pongdam > Birds >    Migration

Take a tip from the original frequent fliers
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 snow and ice. 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?

...Could you hold off on the questions for a moment so I can dish out some answers?

Navigation
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

• Smell

• And probably following their neighbors (many birds migrate in large flocks)

Question: 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.

Flight strategies
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:

                                      Arctic terns can migrate as far as 20,000 miles per year.
 

• 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 about 520 of the 650 bird species that nest in the United States migrate south to spend the winter?
  Because they get bored shivering in the dark. And because it's bleak in the winter. And because there's nothing to eat. And because their ancestors did it.

• 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.)

The world's largest migration of raptors
(hawks, vultures, falcons, kites and other species) flies across Mexico's Caribbean coastal plain near the city of Veracruz.

Raptor-watching is an old specialty among birders--and thousands of eager birders flock to Cape May, New Jersey to watch 50,000 of these elegant fliers make their way between wintering and breeding grounds.

Photo courtesy of the Turkey Vulture Society.

But that is small potatoes compared to the Veracruz migration, where 100,000 birds a day is not unusual. Much of the credit for discovering this abundance of bird life goes to Ernesto Ruelas, a Mexican biologist who has spent 10 years counting this River of Raptors.

Ruelas, who directs the Veracruz office of Pronatura, an environmental organization, credits the geography for funneling 3.3 million raptors through Veracruz each year.

The funneling results from the migratory habits of the raptors. Most of the raptors are soaring birds, so they depend on thermals to obtain altitude. Since the open sea does not have thermals, they are restricted to the Mexican peninsula as they fly between North America and points south. But they can't find good thermals in the mountains which occupy much of Mexico west of Veracruz--meaning they must stay near the coast.

As a result, virtually the entire population of some raptors is flying through a strip of land just 25 miles wide, and that means the countryside is critical for survival of those species. "This is a site that is capable of monitoring continental or worldwide populations of certain raptors," says Keith Bildstein, research director at in Sanctuary in Pennsylvania.

Like the monarch's wintering grounds, the raptors' migration pathway points out how a continent-wide population can depend on small tracts of land. Want to read about the decline of songbirds which depend on isolated patches of summer habitat?

"Courtesy University of Wisconsin Board of Regents."