In progress
12/09/2002
The Biogeography of
Bald Eagles (Haliaeetus leucocephalus)
by Michael Rochelle, student in Geography 316
Thank you for visiting our site. This web pages was written by a student in
Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD.
All photos and maps are posted with specific copyright permission for the
express use of education on these web pages. The students have tried to be as
accurate as possible with the information provided and sources and references
are cited at the end of each page.
Species Name:
Haliaeetus leucocephalus
Photo by David and Valerie Peters
at
www.tgrsolution.net/zoo
Kingdom: Animalia
Phylum:
Chordata
Class: Aves
Order: Falconiformes
Family:
Acciptiridae
Genus: Haliaeetus
Species: Haliaeetus leucocephalus
Haliaeetus
leucocephalus’ most common name is the bald eagle, although, far from being
bald, the taxonomic
name literally means white-headed sea-eagle, so bald eagle is a misnomer based
upon its appearance (Weidensaul, 1996). The bald eagle represents power,
freedom, and for some, spirituality and pristine wildness. It was due to its
strong image that Congress officially adopted the bald eagle as the symbol for
the United States in 1782, and you will find bald eagle emblems on currency,
coins, and the US seal (Stalmaster, 1987). Ironic, that what is revered
symbolically was almost brought to the point of extinction in the last century,
that our ideologies
regarding nature don’t necessarily correspond to our actions that effect it.
Description of Species:
The bald eagle measures from 30 to 43 inches in length, and 70 to 96 inches in
width with a wing span
from 23 to 25 inches. The females tend to be a little larger than the males. Its
feathered body is white in
color on its head, neck, and tail, while the rest of the plumage is dark brown. The bald
eagles bill, talons, and iris are yellow. Young bald eagles plumage is
completely dark brown until they
reach maturity and gradually gain their white feathers from ages three to five.
Although scientists do not
know why they have evolved with the white head, it has been suggested that their
conspicuous appearance
signifies sexual maturity and that it may help serve as a warning to other
animals that they have
occupation and dominance of that territory (Stalmaster, 1987).
Natural History:
The bald eagle was formerly widespread all over North America, but now can be
found mainly along the Pacific
Northwest with much more sparse populations through the interior to the East
Coast (Stalmaster, 1987). You
will find bald eagles near seacoasts, reservoirs, lakes, and streams where fish
populations are abundant
enough to support them. They prefer large and tall trees with open edges (both
broadleaf and pine
needle), so as to have easy accessibility and surveillance of prey (INRIN,
2002). Most often they
will choose to roost and nest in the tallest tree in the area that is closest to
the water. Tall, dead
trees or burned up trees will also make a good spot. During cold seasons they
may seek more sheltered trees
or floodplains surrounded by river bluffs for less severe temperatures. The bald
eagles aggressive
behavior near nest sites indicates that the breeding pair establishes a
territory with a specific range
where they have monopoly over the resources. The average territory ranges from
one to two square
kilometers, and their biggest competitors are other bald eagles due to scarcity
of resources and the fact
that species of their own kind fill the same niche (Stalmaster, 1987).
The bald eagle primarily feeds on fish,
especially
ones small in size that they can swallow whole. They
will also eat small mammals, small birds, injured
waterfowl, and carrion when abundance of fish is not
available (INRIN, 2002). Young bald eagles eat the
same as adults. An interesting adaptation the bald
eagles have acquired for the purpose of becoming great
hunters is their vision. Their eyes occupy most of the
space on their head, have both monocular and binocular
vision, and can see three to four times greater
distance than humans making vision their most
important sense (Stalmaster, 1987). Their feet, talons, and beak are also
adapted for efficiently tearing and killing slippery prey.
Bald eagles are monogamous and establish a pair bond for as
long as both partners are alive (INRIN, 2002).
If one of the eagles dies, then the widow will take a new mate, sometimes in as
little as three days. Trio
bonds have also been known to form, but this is an anomaly, as well as an adult
taking an immature mate
(Stalmaster, 1987). Sexual maturity is reached at ages five to six, and
courtship is done by rituals, such as
vocal displays, chase displays involving acrobatic aerial dives and rolls, and
the most spectacular,
roller- coaster flights where the two birds dive at great speed, doing
somersaults while talons are locked
together, then swinging up again before hitting the ground and repeating the
process (Stalmaster, 1987).
Copulation only happens after the couple has lived together for awhile, and is
initiated by the female
when she goes into a solicitation posture and calls over to the male for
service. Copulation lasts from 15
seconds to two minutes, but can occur several times a day (Stalmaster, 1987).
They breed from February to
March, and lay two to three eggs in March to April with an incubation period of
35 days. Nests are built
in the same spots each year, they build the new on top of the old. Nests are
built from sticks, grass,
leaves, needles, and herbs (INRIN, 2002). Young stay in nests for up to thirteen
weeks and begin flying at that time, however, the adults continue to protect and
feed the young. Bald eagles are territorial during all
the stages of rearing the eaglets. Other duties include sheltering from weather
and guarding from
predators, and both parents take on the responsibilities (INRIN, 2002).
Distribution:
Like most diurnal species in the order Falconiformes, Bald eagles are restricted
to a single region
(Weidensaul,1996). Their indigenous region is North America with occasional
stray birds making it to
Siberia and Greenland. Their northern most extent ends with the Arctic tundra,
because they need trees for a
suitable habitat, while their southern most extent is Baja, and across the
southern border of the U.S.
(Stalmaster, 1987). Why they are limited to this region is probably best
explained by their evolution.
Historically, bald eagles were widespread throughout North America and occupied
a variety of habitats,
including deciduous forests, redwoods, coniferous forests, coastal cliffs and
valleys, as well as inland
cliffs and valleys (Detrich, 1983). Their range and population began to shrink
as soon as European
settlement kicked in, the pre-1800 population is estimated at a quarter million
while current estimates
are at about eighty thousand (Weidensaul, 1996). They once existed in every
state, but now eighteen states
are left with no breeding pairs (Salmaster, 1987). An interesting example of
their habitat fragmentation is
that in California three quarters of their nests are near reservoirs or dams (Detrich,
1983). Although bald
eagles have a continuous distribution within a single region, human impact on
their niche has chopped it up
and made it more of a disjunct pattern. The restrictions to bald eagle
distribution are abundant food supply, which is the most important, but also of
importance is the need for old- growth forest and freedom from human
disturbances (Stalmaster, 1987). One way of adapting to seasonal change in food
supply is migration. Birds that nest in the greater part of Canada during
breeding season will migrate south into the states during fall, because lakes
and rivers freeze during winter in the harsher climates of their range making
food supply dwindle (Stalmaster, 1987). Not all eagles migrate, in the milder,
year round climates along the coastlines there are birds that stay stationary
throughout the year, as well as birds that only nest there or only winter there.
This
range runs from Northern California all the way up the coastline to Alaska, and
on the East Coast, practically all of Florida, then a disjunct pattern all the
way north to Newfoundland. What is interesting about
this distribution is that the breeding range of the southern areas, and the
wintering range of the northern areas remain exclusive of each other, except at
the much smaller regions along the coasts, where you will also find every non-
migratory bird, as well as the biggest population concentrations. As far as
particular areas of concentrations, Alaska has the most impressive with a
thirty- five to forty- five thousand population of wintering eagles (Stalmaster,
1987). The most important single sight is along the Chilkat River in Southeast
Alaska where you will find four thousand eagles feasting on spawning salmon
within forty miles of each other in November. The second biggest concentration
will be found in British Columbia where over twenty- eight thousand
eagles during winter (Stalmaster, 1987). The largest winter population in the
United States is found in
Washington with close to fifteen hundred birds, primarily in the protected
Olympic National Park. Many
bald eagles also winter along the Mississippi River in Missouri and Illinois
where you will find up to a
thousand. Other worthy mentions will be at McDonald Creek in Glacier Park,
Montana, where over five
hundred birds winter, and in Florida where there is over one thousand wintering
eagles and the largest
population of breeding pairs in the forty- eight contiguous states at over three
hundred. As far as
concentrations closest to California, you will find over five hundred in the
Klamath Basin of Southern
Oregon and Northern California, and small populations of wintering and non-
migratory birds in Shasta and
Trinity national forests (Stalmaster, 1987).
Evolution:
All birds, which compose the class Aves, arose from reptiles. Whether they
came from crocodilelike
thecodonts, which are ancestors of dinosaurs, or from small dinosaurs themselves
is still debated in the
scientific community (Weidensaul, 1996). The best fossil example of an
intermediary phase between
reptile and bird comes from the Jurassic period with the species Archaeopteryx,
which had feathered wings
and tail with reptilian teeth and clawed wings. Archaeopterx died out 65 million
years ago at the
great extinction, but with dinosaurs out of the way, other bird species that had
already evolved were now
free to expand during the Tertiary period (Weidensaul, 1996). The first modern
raptors, whose diurnal members
make up the order Falconiformes, appear in the fossil records from the Eocene
epoch about 50 million years
ago. From this epoch also evolved the ancestral kites, which made up the family
Acciptiridae, and the genetic
framework for the 205 species in that family today (Stalmaster, 1987). Included
in this family is the
genus Haliaeetus, which the bald eagles, along with all eight sea and fish
eagles, belong to. The sea
eagles of this genus probably came from the scavenging Asian and Australian
kites of genus Haliastur, and the
first sea eagle appears 25 million years ago (Stalmaster, 1987). We do not know
how far back bald
eagles go, but they have found fossil remains in La Brea tar pits that date back
over one million years
ago. The eight species under the genus Haliaeetus represent a regional
distribution that is almost
absolutely exclusive of each other. Plate tectonics probably doesn’t explain
this, because the eagles are
great dispersers and continental separation had already taken place before they
branched out. What
their distribution suggests is adaptive radiation and, because of the similarity
of ecological niche, maybe a
degree of ammensalism helped evolve the eight separate species.
Other interesting issues:
As soon as Europeans settled in the New World and expanded West, the bald eagle
population began to
decline. Coexistence was achieved to somewhat of a degree in certain areas, such
as Florida, but in other
areas, primarily the ranching parts of the West, bald eagles habit was lost and
they were directly killed,
because they were seen as a threat to livestock (Weidensaul, 1996). Their
diminished numbers were
recognized by the National government, and the Bald Eagle Protection Act was
passed in 1940 which made it
illegal to kill, harm, or harness any bald eagles, dead or alive, including
eggs, feathers, and nests
(USF&WS, 2002). Their recovery after this act passed was short lived, because a
new threat shortly after
began to kill them, that of the chemical pesticide DDT. DDT began by
contaminating their food supply,
then gradually started accumulating in the fatty tissue of female eagles, which
eventually led toreproductive failure through eggshell thinning (USF&WS, 2002).
Things began to turn around in 1972, when DDT was banned, and the Endangered
Species Act was passed, where the bald eagle was one of its first members. Due
to the chemical ban, expanded efforts to locate and understand them, habitat
management plans, protective zones, and season closures, the bald eagles numbers
in the 48 contiguous states have risen nine times from the 1963 population (Detrich,
1983 and USF&WS, 2002). They were originally classified as endangered in 1972,
but in 1995 were reclassified as threatened, making the bald eagle one of the
Endangered Species Acts biggest success stories.
Although, this does not mean that the bald eagle is free from danger, or that it
has actually recovered.
The bald eagle is still just as much threatened by the same thing that has
always threatened them, that is the continually expanding human alteration of
the natural environment.
Bibliography:
-Detrich, Philip J. 1983. “Status of the Bald Eagle in California.” Pg.80-83. In
Nancy Venizebs and Celeste Grijalva,eds. RAPTORS. San Francisco Zoological Society.
-Illinois Natural Resources Information Network (INRIC) / Bald Eagle/
www.inhs.uiuc.edu/chf/pub/ifwis/birds/bald-eagle.html/ 10-10-02.
-Stalmaster, Mark V. 1987. THE BALD EAGLE. University Books, New York.
-U.S. Fish and Wildlife Service/Bald Eagle/http.endangerd.fws.gov/i/b/msabOh.html/11-1-02
-Weidensaul, Scott. 1996. RAPTORS: THE BIRDS OF PREY.Lyons and Burford
Publishers.
send comments to bholzman@sfsu.edu
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