Where and When do birds
migrate?
Approximately 1/3 of all species
of birds migrate twice during the year: in the fall, they leave their
breeding grounds and head
to warmer climates where they spend the winter, and in the spring, they
return to their breeding grounds. The migratory
route of a bird shows the general path of flight that is annually
followed by the bird on its journey between non-breeding and breeding
grounds. Most birds fly in a north-south direction. Neotropical
migrants spend the spring and summer in North America, and the fall
and winter in Central and South America. Below is a range
map of the Wood Thrush, a Neotropical migrant, showing where it
spends its time at different times of the year.
Wood
Thrush Range Map
 
WoodThrush
 Paleotropical
migrants spend the spring and summer in Europe and northern Asia, and
the fall and winter in more tropical climates such as Africa and southern
Asia. The Lesser Kestrel, a paleotropical migrant, spends the fall and
winter in South Africa, and the spring and summer in Europe and the
Middle East.
Lesser Kestral
The
birds of a particular area can be divided into 5 general categories
based on their migratory habits:
Residents: Birds
that live in an area year-round
Summer
Visitors: Birds
that breed in an area in spring and summer but migrate to other areas
for the winter
Winter
Visitors: Birds
that are in an area only for the winter and then migrate elsewhere to
breed
Passage
Migrants: Birds
that pass through an area during the course of migration, but do not
spend a significant period of time there. Areas where birds rest and
feed during the course of migration are known as stop-over
areas or staging areas.
Accidentals: Birds
that may or may not arrive in an area and whose arrival cannot be predicted. They come to the area at irregular intervals,
usually because they have gotten off-course on their way to some other
place.
A species may belong to more than one of the above groups. For example, some populations of the Peregrine
Falcon in North America are summer visitors, and some are residents year-round.
Most birds migrate at
night – this is because they are safer from predators at night, and they
do not have to worry about overheating during the heat of the day.
However, large birds like many raptors, storks, and cranes, must
migrate during the day in order to take advantage of the thermals,
columns of rising air that are produced when the sun warms the ground. As the warm air rises, these big birds can
save energy by gaining lift from riding on these thermals. Birds that take advantage of thermals to achieve
lift are called “soaring birds.”
Many small birds prefer
to migrate over water rather than over land, since they are safer from
predators. However, this requires
them to fly over very long distances without being able to stop and rest.
This is why they have to build up a lot of fat in their bodies
before they begin their journey. The
weakest birds are seldom able to make the full journey.
Some animals don't migrate
south for the winter but do wander around considerably. Below is an animation
of the movements of a Snowy Owl in the Arctic. The animation also shows
the extent of sea ice. Scientists used this animation to try to determine
if the movements of the owl were related to sea ice movement.
Snowy
Owl Video
The snowy owl video was
prepared by David C. Douglas, Research Wildlife Biologist, USGS Alaska
Biological Science Center
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