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)
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.
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.)
From the whyfiles.org
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March and April