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Bald Eagle Facts
Q&A with Jim Watson in 1998
Washington Department of Fish and Wildlife

Characteristics

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 can 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 (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 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.


Life Cycle

Q. How many eggs does an average bald eagle lay in a lifetime?

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, 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 lifetime.


Ecology

Q. How many Bald Eagles are there?

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 eagles 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.

If they are going to nest successfully, there a few things eagles have to do wherever they nest:

  • Build or refurbish their nest (may take only a day, but we'll say 1 week, 7 days)
  • Incubate eggs (35 days),
  • Raise young to independence (perhaps 120 days).

Adding these numbers up comes out to 162 days or a minimum of about 5 1/2 months on the breeding area. More typically, in temperate areas such as Washington state, the adults will remain on their territories at least 9 months of the year before fall migration.


Conservation

Q. When did the bald eagle get on the endangered list? 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.



Jim Watson
Washington Department of Fish and Wildlife
Spring, 1998

 

 

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