Bald Eagle Frequently Asked Questions: Students' Questions and Experts' Answers

Bald Eagle Facts
Q&A with Peter Nye in 2005
New York Department of Environmental Conservation

A: Bald eagles generally weigh between 4 - 6 kilograms, although some have been found both below and above this range, with some Alaskan eagles recorded with weights of well over 7 kg. Female eagles are the larger and heavier of the sexes. Now that we know how much eagles themselves can weigh, we can use that to define how much they can lift. Of many prey items found in nests and weighed, a good general rule seems to be that eagles can carry up to half of their own weight. This obviously means female eagles are able to carry larger prey than the males. Sometimes, eagles have trouble judging the weight of prey. I've witnessed eagles in Alaska "lock on" to a large salmon, obviously heavier than could be carried away in flight, however the eagle is very capable of floating and "swimming" to shore with its prey, rather than give it up. Then, dragged it up on shore, and the feasting began.

Q: I have noticed in various books with photographs of Bald Eagles that their eye color can vary. I've seen blue, yellow, and brown. Why?

A: Generally, eagle eyes are pretty consistent in color. Nestling eagles eyes are nearly black. Juvenile eagles (first year birds just out of the nest), have brown eyes (which can vary in how light or dark they are, but usually they are pretty dark).
As they become immature eagles (ages 2,3), their eye lightens to a light brown. As they get near sexual maturity (age 4,5), their eye turns yellow, and again can be in various shades of lighter to darker yellow, but usually quite light yellow. I believe that the darker eye color of juveniles and immatures may be a defensive mechanism, not seen as the threat yellow, adult eyes might be. Similar coloration and gradual color shift to lighter and brighter are found in the bills of bald eagles as they age. I've never seen blue eyes in eagles though!

A: The life span of eagles in the wild is generally around 30 years. Actually, little is known about the reproductive life of eagles as they age, due to the lack of known-age/banded birds and intensive observations of same. I can tell you that we captured one of our local breeders at her age 25 years, and she went on to breed and raise young successfully in her 26th year. It is my opinion that eagles are probably productive until they die. It would be mal-adaptive for adult eagles to remain in the population as non-contributing members. More often, I believe what happens is the aging/unproductive bird is actually killed and replaced by a younger, more productive and fit adult.

Q: We know that dog life spans are 7 years to 1 human life span, so what is the eagle's life span to a human span?

A: To answer that we have to explain how long eagles can live.
In captivity (a more coddled life...), bald eagles have lived well into their 40's. But in the wild, their life is undoubtedly much shorter, either cut short by human beings, by other eagles, or by the rigors of their life. In the wild, we believe eagles live around 30 years. Therefore, I guess you'd say an eagles life is about 2.5 to each human year, based on our current average life expectancy.

Q: Do they reach a point like humans where they cannot bear young?
For 3 years I have observed a nesting pair of Bald Eagles near my home. The pair has been nesting for 15 years in the same location. Last year the male crushed one egg in mid air. The other made it to a first flight only, never to be seen after a few days. It stayed in a tree near the nest, but then died. The pair is currently nesting. She laid the eggs on 2/28.

A: I'm curious to know where you live! Judging by the February 28th egg date, you must be in PA or south NJ? The mid-air egg-crushing you mention is quite strange and begs another question. Did this pair raise/fledge any young the same year? I think, you are saying one young was fledged. Often, one of the adults will remove egg-shells from the nest after hatching; could you simply have seen egg-shells being "cleaned" out of the nest and dropped? Adults will also sometimes remove whole eggs that don't hatch, fly from the nest with them and drop them (they will also simply eat them in the nest). I have never heard of anyone witnessing "crushing" of an egg in mid-air. Perhaps it was one of these normal behaviors you witnessed. I have no idea what could have happened to the fledgling. Again, after fledging, juveniles will often perch along the shore away from the nest for a long time, in hard to observe places. Were both adults present at the nest the whole season?

Q: We live in Colorado and would like information on the migration path the eagles take in the mountain states and west coast.

A: Colorado hosts a "winter" eagle population much larger than its summer breeding population, and most of these 500-1000 birds hail from Canada. One of the major wintering areas for bald eagles in CO is the San Luis Valley in south-central CO. Back in the 1970's and '80's an eagle researcher named Al Harmata captured wintering eagles in CO and studied their migration. You might want to look up his work (Ph.D. thesis, 1984, Montana State Univ., Bozeman). I believe some of his birds were tracked back to summering grounds in the Northwest Territories of extreme northwestern Canada and Saskatchewan. I would guess based on other studies and banding records that other CO wintering eagles come from British Columbia, Alberta, and perhaps Manitoba. Also, a considerable amount of work on bald eagle migration in the western United States, particularly in Washington State, has been conducted by Jim Watson of the Washington State Department of Fish and Wildlife.

A: You mean besides reptiles? Only half-kidding.
According to Mark Stahlmaster's wonderful book on bald eagles, the "sea eagles" of which the bald eagle is one of eight, began evolving tens of millions of years ago from a group of birds known as kites, which share many of the same characteristics of the sea eagles (fishing, scavenging, breeding). He states these ancient kites provided the genetic framework for an ancient sea eagle, which eventually evolved into eight separate sea eagle species. According to Mark, there are fossil remains of this ancient sea eagle at least 25 million years old. Although we don't know precisely when the white-headed sea eagle (our bald eagle) evolved, there are numerous fossil remains, some from North America as long as one million years ago. Bald eagles are in the order "Falconiformes" (including most raptors except owls), and the family "Accipitridae", which includes 205 species of eagles, hawks, kites, old world vultures and harriers. Four groups of eagles are recognized, containing 59 different species of eagles worldwide: Buzzard-like eagles (Harpy Eagles), Fish and Sea Eagles, Snake Eagles, and Booted Eagles. Our golden eagles are members of the "booted" eagles.

A: As mentioned above, bald eagles have been found in fossil remains in North America more than one million years old, including in tar pits in California and frequently in Indian middens throughout the continent. The bald eagle is exclusively a bird of North America. I'm not sure what you mean exactly by "where were the first sightings", but I would bet that as early man crossed the land bridge from Asia through Alaska and migrated south, that they encountered eagles along the coastal areas not covered in ice. I'm sure native Americans living in New York and the east frequently encountered bald eagles, and surely captured and killed many for food and spiritual purposes. Later than that, on his journey up the Hudson River in 1609, I'm sure Henry Hudson was privileged to observe bald eagles along the pristine shores of our Hudson River.

A: Golden eagle A20 was about 2 years old when he was captured, and is now 5 years old. Bald eagle V98 was a full adult when we captured it last March, so it was at least 5 years old. We cannot say how old an adult is once it obtains its white head and tail (adult plumage), unless it was previously banded. We recently trapped an eagle we had banded in 1976, at 25 years of age.

A: In New York State, in 2004, we confirmed 84 breeding pairs of bald eagles; 79 of these pairs laid eggs (exact number unknown), and only 66 of these pairs were successful (fledged young). A total of 111 young were fledged, meaning that, on average, each of the 66 pairs fledged 1.68 young. Biologists believe that mostly, bald eagles lay two eggs, even if only one eaglet is hatched and fledged. Of course, without numerous intrusions into the nests, we can't know exactly how many eggs are laid by every pair. Of the 13 of our 2004 pairs that failed, they laid at least 13 eggs, and possibly as many as 26. About 5 percent of the time, eagle pairs will lay 3 eggs. In addition, we recovered 10 unhatched eggs from 8 of the 66 successful nests. Every time I visit a nest to band young, I dig around the nest cup in search of buried, unhatched eggs. These are valuable for contaminant and shell-thickness analysis. So, at least 23 more eggs were laid, and probably more, than the 111 young we had this past year. Biologists believe that if eagles produce at least 1 young per breeding attempt, that the population will do fine and not decrease.