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Migration
Basics
Migration
is a fascinating aspect of animal ecology. Migration inspires us
whether we are studying salmon migrating thousands of miles back
to their spawning grounds, huge flocks of sandhill cranes migrating
across the northern skies, or caribou crossing rivers in the fall.
Migration
has captured the interest of humans for centuries. Ancient civilizations
devised many myths to explain the periodic appearance and disappearance
of vast numbers of animals. For instance, people once thought that
tiny birds called swallows buried themselves in the mud at the bottom
of lakes to get through the winter. Instead, scientists found out
that swallows fly all the way from Europe to Africa and back in
one year. Perhaps the truth was harder to believe than the myth.
What is
migration and why do it?
Animals that live
in habitats that are difficult to survive in year round, must evolve
a way to cope with the difficult time of year. A strategy used by
many mammals and other species is hibernation. Migration is another
option for animals that can move across long distances. They survive
by leaving the area for part of the year or part of their life,
and move to habitats that are more hospitable.
The
most common reason to migrate is to take advantage of food, shelter,
and water that vary with seasons, or life stage. The availability
of food and water can change throughout the year. For instance,
the lack of insects and leaves in the winter means there is less
food to eat. Some environments have a rainy and a dry season that
are very different. Temperatures change between the seasons, some
areas getting very cold or very hot which can be hard on some species.
Sometimes it is not about getting food but about staying safe. Deep
snow may make animals easier to catch by predators, or animals may
go to special breeding grounds to keep their young safe when they
are especially vulnerable.
Types of migration -
there are lots of different kinds of migration. These terms are
used to describe attributes of migration such as timing, direction,
the reason for migration, and how many of the species migrate. More
than one term can be used to describe one species migration patter.
Some common types of migration are:
- Seasonal migration
- is migration that
corresponds with the change in seasons. Most migration fall within
this category. Many altitudinal, longitudinal, latitudinal, and
reproductive migrations take place when the seasons change.
-
- Latitudinal migration
- is the movement of
animals north and south. The geese flying south for the winter
is one of the most recognizable examples of latitudinal migration.
By moving north and south, animals are changing their climate.
In the northern hemisphere, the winters are colder as you move
north and warmer as you move south. On the other hand, summers
in the north can be rich in food, especially in the far north
where summers are short, but the days are very long.
-
- Altitudinal migration
- is the movement of
animals up and down major land features such as mountains. While
food may be plentiful in alpine meadows in summer, the winters
will be colder and have more snow as you move higher up. Many
animals take advantage of the summers, and then move to lower
more moderate elevations during the winter.
-
- Reproductive migration
- is the movement of
animals to bear young. The area may be safer for the young because
of fewer predators or more shelter from predators. In other cases,
the area is safer because the animal requires a different type
of habitat when it is young than when it is older.
-
- Nomadic migration
- is the movement of
animals not between known areas, but it looks to us more like
wandering. Grazing animals will move across larger expanses as
the grasses get eaten and they travel to greener pastures.
-
- Removal migration
- is the migration of
animals that don't come back. This can be when resources such
as food, water or shelter are no longer available to animals where
they are. The environment can have changed, through fire, flooding,
invasive plant species or human development or other causes and
the animals need to leave to survive. Another cause of removal
migration is when the resources haven't changed, but the population
gets too big, there are too many animals and many of them leave
to find food, water and shelter elsewhere. Removal migration is
what brought immigrants to America in the 1800s.
-
- Complete migration
- is when virtually
all members of the species leave their breeding range during the
nonbreeding season. Many North American birds are complete migrants.
Most complete migrants breeding in northern temperate and arctic
areas (such as Alaska) of North America, Europe, and Asia. Complete
migrants travel incredible distances, sometimes more than 15,000
miles (25,000 kilometers) per year. The wintering areas for most
complete North American migrants are South and Central America,
the Caribbean basin, and the southern most United States.
-
- Partial migration
- The most common type
of migration is partial migration. Partial migrant means that
some, but not all, members of a species move away from their breeding
grounds during the nonbreeding season. There is an overlap between
breeding and nonbreeding ranges of the species. Species like Red-tailed
Hawk, Herring Gull, and Golden Eagles are partial migrants over
much of their North American range.
-
- Irruptive migration
- Migrations that are
not seasonally or geographically predictable are termed irruptive.
Such migration may occur one year, but not again for many years.
The distances and numbers of individuals involved area also less
predictable than with complete or partial migrants. In some years,
irruptions can be over long distances and involve many individuals,
or they can be short and involve only a few.
Migration Examples
- Humpback whales
of the Pacific Ocean head south in the fall to give birth to their
young in subtropical waters off Hawaii, and then in late spring
head north to spend the summer in the cold waters off Alaska that
are rich with food.
- Salmon are
reproductive migrants that start their lives in freshwater streams,
move to the open ocean for their adult lives, then return to their
home stream to lay eggs.
- Dall sheep
of Noatak National Preserve are seasonal, altitudinal migrants
that spend summers near the top of mountain ranges and then winter
at lower elevations where there is less snow and food easier to
find.
- Arctic terns
are complete migrants that spend all year in summer by alternating
subpolar regions in the northern and southern hemispheres.
- Golden eagles
of Denali National Park and Preserve spend the summer in the north
where there is plenty of food, and head south for the winter when
there is less food in the north and the temperatures drop far
below zero. While all of the golden eagles of Denali do migrate,
golden eagles are considered partial migrants because those that
live far enough south do not migrate.
- Sea turtles
return from ocean waters to the coast to lay eggs in the sand,
where they hatch and head to the open ocean until it is their
turn to lay eggs. They are another example of reproductive migrants.
- Locusts change
when they get too crowded and become more active and social creating
large groups of insects that move across the land in search of
new places with plenty of food (and fewer locusts). This adaptation
to overcrowding is removal migration.
- Great gray owls
are an irruptive migrant, migrating southward only occasionally
and in numbers that vary greatly. Northern finches and crossbills
are also irruptive migrants.
Migration
cues
How
do animals know when to migrate? That depends on the type of migration.
For many types of migration it is the change of seasons that spurs
animals on. As summer becomes fall, days become shorter and that
can trigger animals to prepare for migration. Closer to the equator,
the days don't change in length and one theory is that animals become
restless after too many days with a constant length.
Other
migrations are initiated by seasonal conditions. Food availability
can be a motivator for some longitudinal and altitudinal migrators.
For example, as plant foods in upper elevations become hidden under
snow, animals move down toward the valleys, and then in the spring
as the plants come out again, animals move back into the upper areas
following the plants as they appear. Nomadic animals move to the
next feeding ground as they run out of food where they are. As ponds
dry with seasonal changes, animals will move to find available water
supplies, and then return during with the seasonal rains.
In
some species, migration happens when there are just too many animals
too close together. The overcrowding causes many of the individuals
to leave in hopes of finding another habitat with less competition.
Or when there isn't enough food, not because of the changing seasons,
but because the food where they are has been eaten. Then the animals
start moving in search of new food.
Navigation
How
do animals know how to get where they are going? That depends on
the animal and where it is going. There is strong evidence that
genetics plays a large role in migratory behavior and that animals
inherit migratory routes from their parents genetically.
Animals
use a variety of different information and senses to navigate. Researchers
believe that most animals use a combination of navigation cues,
depending on where they are and what the conditions are. In
shorter migrations, animals do not need complicated navigation abilities.
They can simply follow the food or the water, or head downhill to
the valleys in winter and back up toward the ridges in summer.
Researchers
have learned a lot about animal migration by studying animal movements.
Starlings, for instance, orient themselves using the sun, compensating
for how the sun moves across the sky throughout the day. Mallard
ducks can find north using the stars of the night sky. Animals as
diverse as migratory birds, salamanders, salmon, or hamsters use
the geomagnetic field for orientation. Studies of loggerhead turtles
revealed that hatchlings have the ability to sense the direction
and strength of Earth's magnetic field, which they use for navigating
along the turtles' regular migration route. Scientists have discovered
a collection of nerve cells in the brains of subterranean Zambian
mole rats that enable the animal to process magnetic information
used in navigation.
Animals
can also use mental maps. Just like people they become familiar
with an area and navigate their way using land features like mountain
ranges, coastlines, rivers, and even, in the case of dolphins, the
shape of the sea floor.
Smell
can be a powerful tool for many animals. Many land animals can create
mental maps based on the smell rather than the just sight of major
land features. Salmon use smell to find the exact stream that they
were born. Fish
can use water currents that circulate around the oceans, or they
can swim against the current or with the current in streams and
rivers.
Adaptations
for migration
Migratory
animals that travel long distances have special adaptations to help
them get there. The most obvious are birds. They have wings that
allow them to fly long distances, their bodies are especially light
(they have hollow bones) so they can stay high in the air, and they
don't have unnecessary weight to carry around. Geese fly in formation,
the shape of a "V", which decreases the wind drag on all
the birds along both sides.
Birds
add on extra fat stores to give them enough energy for long flights
north and south, because they do not eat during the migration. Similarly,
whales stock up well on food in the northern seas before heading
south for the winter, because they don't eat on the way.
Land
animals must rely on their legs and feet to get them where they
need to go.
Sources:
Carol McIntyre, personal
communication.
Baker, R. Robin. 1978.
The Evolutionary Ecology ofAnimal Migration. Holmes & Meier
Publishers, Inc. New York.
Animal Migration. 1993-2002.
Encarta.com, Microsoft Corporation . http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761557464#s12
Able, Kenneth P. 1999.
Gathering of Angels, Migrating Birds and Their Ecology. Comstock
Books, Ithaca and London.
Kerlinger, Paul. 1995.
How Birds Migrate. Stackpole Books, Mechanicsville, PA.
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