Take a tip from the original frequent
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?
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)
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
Second, star navigation changes as
new constellations appear on the horizon as the birds travel north
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
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
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
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
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.
Arctic terns can migrate as far as 20,000 miles per
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
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
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.
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
Ruelas, who directs the Veracruz
office of Pronatura, an environmental organization, credits the
geography for funneling 3.3 million raptors through Veracruz each
The funneling results from the
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.
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
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
which depend on isolated patches of summer habitat?
University of Wisconsin Board of Regents."