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Denali
Golden Eagles and Migration
Review
Migration Basics
for background to golden eagle migration.
When do
Denali golden eagles migrate?
Adult eagles arrive
in Denali National Park and Preserve by late March or early April,
although some may come back earlier. Subadults (only 1 year old)
come back later; they may not even start the return migration north
until late April.
Golden eaglets usually
hatch in mid to late May or early June. When the chicks are about
70 -75 days (not quite 2 1/2 months) old they are ready to start
learning to fly. They fledgings (eaglets learning
to fly) remain in their natal areas (the area where they
were born, their nest area) for four to eight weeks after fledging.
During this time, they are dependent upon their parents for food
and protection from predators and they spend most of their time
exercising their muscles and learning to fly. They learn to fly
independent of each other and their parents. In late September or
early October, when the eaglets are about 4 months old, they leave
their natal areas and begin their southward migration completely
independent of their parents. The success of their trip rests on
their adaptations to migration.
Where do
the eagles go?
Our
telemetry (radio tracking) studies indicate that many of
the golden eagles of Denali spend winters in the Lower 48 states
of the US. They may travel as far east as Kansas and South Dakota
and as far south as northern Mexico. Many of Denali's golden eagles
spend their winters along the Rocky Mountains of Montana and Wyoming
and Colorado.
Our
research suggests that the fledglings do not leave Denali when their
parents do, and the eagles travel alone. The fledglings find their
way south on their own. The eagles take similar paths, but no two
studied have been quite the same. How do they know where they are
aiming for? We don't know exactly, but other scientists suggest
that genetics plays a large role in migration.
It
takes most juvenile golden eagles from Denali from four to nine
weeks to complete their first southward and northward migrations.
Adult golden eagles probably migrate faster and take more direct
routes than do juveniles and subadults.
What adaptations
do golden eagles have for migration?
Golden
eagles, and all other migratory birds, have physical and behavioral
adaptations for migration. The physical adaptations are how their
bodies are designed to make long flight possible, and the behavioral
adaptations include how the eagles use their physical adaptations.
Physical
Adaptations: Golden
eagles are well adapted to flight. The eaglets have obviously adapted
for rapid growth so that within four short months they progress
from being an egg to migrating upwards of 2000 to 4000 miles. Their
lungs and the hemoglobin in their blood are very efficient and help
them get extra oxygen to their muscles. They usually have a lightweight,
yet strong, skeleton with hollow bones. Their internal organs are
smaller to lighten their body weight. Even with these amazing adaptations,
flight is one of the most costly means of transportation and there
are other adaptations necessary for migration.
Gliding and powered flight (flapping) are the two
basic types of bird flight. Gliding flight, where the wings are
held outstretched from the body and no flapping occurs, is the simpler
and uses less energy. The wings are moved so that more or less surface
area is exposed during a glide. Soaring flight is gliding in circles.
The way that a gliding or soaring eagle gains altitude is to find
updrafts, including thermals - warm air that rises and can
carry the bird along with it. In powered flight, the bird flaps
its wings up and down to provide lift (the force that keeps
the bird flying instead of falling) and thrust (the force
that moves the bird forward through the air). During migration,
golden eagles use both gliding and powered flight.
Flying
uses a lot of energy. Whether or not an eagle is successful in migration
depends on many factors, but it cannot be successful if it burns
more energy in flying than it has stored in fat and can eat along
the way. Powered flight "costs" much more energy than
gliding, and is more costly to large birds than small birds, because
of their larger weight and wing size. This is probably why large
birds use gliding flight as much as possible. Migration using powered
flight almost doubles the amount of energy a bird needs each day.
Wing shape and size also affect the energy cost of flight. Birds
with long, pointed wings generate more power with less energy than
birds with short, rounded wings.
Flight
involves more than simply whether a bird glides or flaps. Wing span,
wing shape, wing area, tail area, tail length, mass, and musculature
are all important in how a bird flies. Wing span is the distance
between the tips of the outstretched wings. Wing area is
the surface area of the wings that comes in contact with the air.
Likewise, tail area is the area of the tail that comes in
contact with the air. Golden eagles have large wings, large tails,
and huge flight muscles in their breast. Because of these features,
they are capable of flying at high altitudes and for very long distances.
During
gliding and soaring, golden eagles constantly adjust their wing
and tail area by opening and closing their wings and tail feathers.
For the slowest glides, as in soaring, the wings are held out as
far from the body as possible to increase surface area. Greater
wing and tail area also increase lift, which is needed at slow speeds.
At faster speeds, the wings and tail are closed a bit, resulting
in a smaller wing span, wing area, and tail area. During the fastest
cross-country glides, the tail is closed and the wings are sometimes
drawn in to just over half of the maximum wingspan. This decreases
the drag that results from breaking through the air with the leading
edge of the wings and tail, and from the friction of air flowing
across the wings and tail.
The
ability of hawks and eagles to use lift explains how soaring migrants
can complete migration even though they may fly slowly. Hawks and
eagles are adapted to use even the smallest and weakest of thermals.
With this ability, they are able to lift off in the morning and
land late in the day, getting in many hours of flight with relatively
little energy. When thermals are not available, hawks and eagles
resort to powered flight. Thus, weather conditions affect migration
strategies of these species. Overall, golden eagles are energy-efficient
migrants and they soar and glide during much of their migratory
journeys by taking advantage of air currents and thermals along
the way.
Energy: The energy needed for migration comes from food eaten
before and during migration. As a bird flies, it first uses sugars
available in the blood and liver, just like people do when they
are exercising. This energy will last for a short time and then
the bird uses its fat deposits. To fly long distances a bird must
carry lots of fat. Gram for gram, fat is the most energy-rich substance
that animals produce and store. For the same weight, fat has about
twice as many calories of energy as the same amount of carbohydrate
or protein. Without fat a migrating golden eagle cannot fly far
or survive long periods of time. The
amount of fat a bird deposits before migration varies by species.
For instance, shorebirds seem to deposit the largest migratory fat
stores with the average being about 66% and a range of 50 to 100%.
This means that some species double their lean weight before migrating.
Hawks and eagles rarely deposit more than about 15% of their body
weight in fat.
Juvenile
golden eagles probably store fat before they leave their nesting
areas while their parents are still providing them with food. Once
they leave their nesting areas they are traveling over unfamiliar
ground and their hunting success is probably low. Because golden
eagles use gliding flight during migration they may not need to
store as much fat as powered fliers like sparrows and geese. When
their fat runs low, they need to find food along their migratory
routes. Researchers suggest that carrion (animals that are
already dead) is probably a common food source for migrating juvenile
golden eagles. If a bird depletes its fat deposits, it resorts to
burning the protein in its muscles. The breast muscle, which is
critical for using its wings, becomes smaller, the bird flies slower,
and eventually it dies.
Behavioral
Adaptations: The behavioral adaptations include type of flight
(flapping vs gliding), flight speed, altitude, flight direction,
and seasonal timing. Migrating birds must decide how fast to fly,
how high to fly and which direction to fly. For example, a golden
eagle must decide when to glide and when to flap. Golden eagles
must decide how fast to fly. If it flies too fast, it will use energy
faster, and it may run out of fat. If an eagle flies too slowly
it may not complete its migration. Studies show that migrating birds
adjust their flight speed to either minimize the energy they use
or maximize how far they travel. By flying faster in headwinds and
slower in tailwinds, a migrating golden eagle can increase the distance
it travels. Golden eagles that complete migration, probably select
speeds that promote a fast yet energy-efficient migration.
Do the
golden eagles return to Denali next summer? Why?
Adult golden eagles that
are members of the breeding population probably return to or near
Denali each year to nest. Their offspring, however, return to Alaska
but generally do not return to the park in their first few years
of life. Golden eagles are long-lived birds and it usually takes
four to five years from them to reach sexual maturity. During their
first years of life, they may travel great distances in search of
food supplies. Once they are four or five years old, they may spend
more time near Denali in search of a mate and a nesting territory.
Sources:
A Gathering of Angels
by Kenneth Able. Comstock Books.
How Birds Migrate by
Paul Kerlinger. Stackpole Books.
Flight Strategies of
Migrating Hawks by Paul Kerlinger. University of Chicago Press.
Carol McIntyre, wildlife
biologist, Denali National Park and Preserve
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