Students' Questions and Experts' Answers
Contributed by Eagle Expert Jim Watson
Washington Department of Fish and Wildlife
Special thanks to Jim Watson for providing
his time and expertise in responding to your questions.
Pigeon River Elem.
Q. When did the bald eagle get on the endangered species
list and when did it get off?
A. On February 14, 1978, the bald eagle was federally listed as
endangered in all of the lower 48 United States except Washington, Oregon,
Minnesota, Wisconsin, and Michigan (it was classified as threatened in
these states). The species was reclassified as threatened in the remaining
states on August 11, 1995. Of course, the threatened status means that
bald eagle populations are increasing, but have not increased to the point
where they are out of danger.
Q. How do you tell the whether the bald eagle is a
male or female?
A. As with any birds where the genders have non-distinct plumages,
the only ways to tell differences in sex are through size
dimorphism (i.e., size differences) or in internal examination
called a laparoscopy . Bald eagles exhibit size dimorphism; females on
the average about 1/3 larger than males. Data compiled by Mark Stalmaster
(1987 - "The Bald Eagle" pg. 16) found that 2 size measurements,
beak depth and hallux (toe claw) length, show the greatest separation
in sexes. These measurements can be used in the following equation: sex
= (bill depth x 0.392) + (hallux length x 0.340) -27.694 (measurements
in millimeters). If the answer is positive, the eagle is a female. If
the answer is negative, the eagle is a male. See if you can figure out
the sex of this eagle -- it is 1 of the 4 birds we are tracking in the
western study: bill depth = 34.5 mm; hallux length = 43.0 mm.
Practically, I can guess the sex of most birds when they are on the bait
and certainly in hand just by general size differences. Birds in the overlap
area of the measurements are more uncertain; a subjective way to sex these
is by temperament; females are docile and don't bite or squirm...males
are very aggressive in hand. I also sex the birds prior to capture by
listening to their calls-- the fluting calls of males is almost a scream,
females is pitched much lower.
Q. What is the average lifetime of a bald eagle?
A. Most of what we know about how long eagles live is from birds
kept in captivity. These birds may live 40 years or longer. Information
from a few wild, banded eagles shows that they may live to be 30 or a
little older in the wild. I suspect that a 25 year-old bald eagle in the
wild is old, and a 30 year old eagle is very old.
Q. How many Bald Eagles
A. A 1975 estimate of the total bald eagles in the world (since
they are only found in North America, I could say North America) was
between 35,000 and 60,000! Most of these are in Alaska and Canada where
bald eagles are not endangered. To give you an idea of how the population
has grown in the lower 48 states, in 1963 there were 417 breeding pairs
known, and in 1994 that number was up to 4,452!
Q. How long do they stay (on nesting grounds) after
they migrate in spring?
A. It all depends on what latitude they breed at. Eagles migrating
to and breeding at northern latitudes (i.e., Yukon Territory) probably
stay a shorter period of time, and have a shorter nesting season than
those at southern latitudes (i.e., California). That is because of the
shorter season in the northern areas. The water stays frozen later into
the spring, and fall comes earlier there.
a few things eagles have to do wherever they nest, if they are going
to nest successfully. They must :
or refurbish their nest (may take only a day, but we'll say 1 week,
eggs (35 days),
raise young to independence (perhaps 120 days).
these numbers up comes out to 162 days or a minimum of about 5 1/2 months
on the breeding area.
typically, in temperate areas such as Washington state, the adults will
remain on their territories at least 9 months of the year before fall
Q. How many eggs does an average Bald Eagle lay in
A. The average bald eagle clutch size is just under 2 eggs/clutch
(1.9). If we assume that a female eagle begins nesting at age 5, and lives
until she is 25 (see above question about longevity), she will have 20
years of egg-laying. There is no evidence that a healthy eagle reduces
egg-laying as she gets older. So 2 eggs/year X 20 years = 40 eggs in her
Washington Department of Fish and Wildlife