Biology Reference
In-Depth Information
CHAPTER 7
Do You Know the Way to San Jose? Bird Migration
Since earliest times, people have wondered about the comings and goings of birds.
Ancient Greeks explained their seasonal disappearance by a belief that they buried
themselves under mud for the winter. People in the Middle Ages thought birds
wintered on the moon. It wasn't until world travel became more prevalent that people
began to comprehend patterns of bird migration, and even in the twenty-first cen-
tury, scientists are still unearthing new mysteries about it. If they still have questions,
is it any wonder that the rest of us do?
The Mysteries of Migration
Q Why do birds migrate?
A
Most birds that breed in the huge landmasses of the Northern Hemisphere feed on in-
sects, fruits, fish, and weed seeds — foods that disappear or are covered with ice or snow
in winter. These birds must migrate south to find suitable food in winter, but they return
north apparently to capitalize on the rich food and fewer competitors during the breeding
season.
Although many people assume that birds migrate to escape cold winter temperatures,
birds can actually survive very cold temperatures, as long as they have enough food. Even
tiny birds, such as Boreal Chickadees, can survive temperatures as low as -50°F (-46°C).
They survive the long winter months by eating spruce seeds and insects they have stored
in the crevices of trees.
Q How far do birds migrate?
A
It depends on the species. Some birds don't migrate at all, such as Florida Scrub-Jays,
which remain near their area of birth for their entire lives. A few mountain species such as
Sooty Grouse just move up and down the mountain with the seasons. Depending on where
they live, individuals of some species, such as American Robins and Dark-eyed Juncos,
may move less than a hundred miles.
Other species travel thousands of miles each year. Eastern Kingbirds that nest as far
north as Canada's Yukon, Northwest Territories, Labrador, and Newfoundland migrate all
the way to South America for the winter. Arctic Terns and many shorebirds that nest on
Hudson Bay or the Arctic tundra winter in southernmost South America, traveling 24,000
miles (38,625 km) each year between their nesting and wintering grounds. Arctic Terns
