Q&A about Bald Eagles with Peter Nye
New York Department of Environmental Conservation

Q: How tall is a bald eagle?
A: About 30 inches.

Q. How many feathers in the average adult eagle?
A. 7,200.

Q. How slowly can an eagle fly and still stay in the air?
A. Very slowly; think about an eagle just taking off. The slower they fly, the more wing flapping they must do to stay in the air and the more energy they burn.

Q. How fast can a Bald Eagle fly at topspeed?
A. Average migratory flight/soaring speed is 50 kilometer/hour; normal flight speed going from perch to perch is much less than this.

Q. What is the exact wing span of an adult bald eagle?
A. Northern eagles range 200-235 centimeters (6-7 feet); your southern eagles are quite a bit smaller in all sizes (weight, wingspan, etc). One of the largest wing-spans on record is of a bird with a 243 cm wing span - 7.9 feet!

Q. What type of feet do bald eagles have?
A. Scaled (featherless) feet with 4 toes, each with a very serious claw (talon). Three toes face forward, a 4th (the hallux) faces backward to aid in gripping prey.

Q. How big is an eagle's brain?
A. It's quite small, about an inch or so sized cube.

Q. How many species of eagles are there?
A. There are 59 species worldwide.

Q. Can eagles fully turn their head around like owls?
A. No, but they do have considerable movement.

Q. How powerful is their eyesight during the night and day?
A. Not so great at night, and about 3-4 times better than humans in day.

Q.Are bald eagles strong enough to carry off babies and small children?
A. No. This is an old wives tale! They can carry only a few pounds at the most.

Q. Can an eagle sweat?
A. No. They "thermoregulate" (control their temperature) by panting with their mouth open or through heat loss through the unfeathered legs and feet.

Q. How do feathers stay attached to the bird? When I find a feather on the ground, it doesn't seem to have any roots.
A. Feathers, like the scales on the feet or the claws or the horny sheath of the bill are keratinous outgrowths of the skin, similar to our nails. Feathers grow out of skin follicles, just as human hair does. The skin tightly grips the feather cone at the follicle and tiny bunches of "feather" muscles in the skin at this site and between follicles holds the feathers and causes their movement. If you have ever tried to pull feathers out of a bird, especially a large wing or tail feather, you know how strongly they are held into the skin which surrounds and grows over the shaft.

Q. Do bald eagles play?
A. Great question! With wildife, it is often hard to determine reasons behind behaviors we may observe. I do believe that eagles get enjoyment out of certain activities, which could be called play, such as when they chase each other in flight, tumble, roll, etc. As with humans, I think immature bald eagles are more prone to "play" than adult birds, who always seem to have something deliberate to do:)

Q. What's the difference between bald eagles and golden eagles?
A. The primary difference is that bald eagles belong to a group of "sea" eagles who live in or near aquatic environments and are piscivorous (fish eaters); Golden eagles belong to an entirely different group of eagles known as true or "booted" (legs with feathers versus scales) eagles and are upland eagles, meaning they are not near water; they hunt upland mammals mostly versus fish. These are just 2 of about 59 species of eagles worldwide, but the only two which we have here in North America (except for another species that occasionally shows up in extreme southwest Alaska). The "bald" eagle got it's name from the old English word "balde" which means white-headed (not hairless!). "Golden" eagles likely got there name from the top and back of their head and neck which are a beautiful golden color.

Q. Do you think that eagles are afraid of people?
A. I think their natural instincts tell them to be cautious and steer clear of humans.

Q. Is there any significant reason why bald eagles have yellow eyes?
A. Great question. I really don't know if there is any reason that the adult iris is light, except that it is a morphological difference with the immature birds which have darker, brown iris' till sexual maturity. Could the yellow eyes look more threatening?

Q. Does a female bald eagle have a white head like a male?
A. Yes, they look identical. The female is larger than the male though, and measurements taken of certain body parts such as the bill and rear claw (hallux talon) can distinguish the two sexes; otherwise you can't tell.

Q. How does an eagle see an animals on the ground while soaring high above?
A. They have extremely keen vision. Their eyes are specially designed for long distance focus and clarity. The eye is large with a large retinal surface area with a high concentration of cones (all of our eyes have rods and cones which allow us to see) which aid in visual acuity and color perception, among other features. It has been estimated that eagles can see 3-4 times farther than humans and that they can see another eagle soaring nearly 50 miles away.

Q. What is the bald eagle's diving speed?
A. Pretty fast when they do it, i'd bet 75+mph, although they seldom really "dive".. They catch prey by flying low and "snatching with their feet mostly, not like ospreys or peregrine falcons that actually dive at their prey.

Q. Can a Bald Eagle swim?
A. Great question. Absolutely. They are very good swimmers, and i've even seen older nestlings who can't fly yet swim. It's not uncommon for an eagle to "misjudge" and latch into a fish too heavy/large for it to fly with, so they then may swim quite a distance to shore (wouldn't want to let go of lunch now would we), drag the fish up on shore and then eat it.

Q. How fast and how far can a bald eagle fly when flying for 30 minutes?
A. That depends on what the eagle is doing. If it is just flying from one feeding area to another or from its nest to the end of the lake say, it probaly flies about 20-30 miles per hour. When migrating, eagles seldom flap their wings; rather, they use thermal updrafts to gain great altitude and the saor in a long, descending glide within which they can hit 50-75 mph easily.

Q: Why are the adult eagles' heads white?
A: That's a great and logical question! While no one knows the answer for sure (not scientifically proven) below are some hypotheses:

  • for visibility, to makes it easy to locate and identify other individuals of the species
  • for denoting sexual maturity, versus immature eagles, which lack the white head
  • for indicating dominance, like top-dog

Q: How can you distinguish between adult male and female bald eagles?
A: Visually, they look identical, but as with most raptors, the female is larger (heavier and bigger) than the male. Sometimes this is clearly visible in a pair, when you see both together at the nest, but otherwise, we are just guessing. In the hand, biologists can differentiate male versus female using two key body size measurements, the depth of the bill (beak) and the length of the hallux talon, on the rear toe. These measurements are plugged in to a neat formula developed by eagle biologist Gary Bortolotti back in the early 1980's, based on numerous measurements of eagles of known sex.

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.

Q. Do eagles molt annually, or how frequently do they shed their feathers?
I have seen references that state they do not molt annually. All chicks grow early feathers, which last during their adolescence. They molt into adult plumage after breeding, and according to Coles B H. (Avian Medicine and Surgery. Blackwell Scientific Publications, 1985) large birds in adulthood such as eagles molt bi-annually. Another reference states that all adult birds molt annually, in a gradual process through spring, summer, and fall, while flight feathers are molted only during July, August, and September. This claim is not substantiated. What is the truth?
A. Believe it or not, this is not an easy question to answer; even with all the years and people studying eagles, the molting process is still not precisely understood. Prior to reaching sexual maturity at about age 5, we need to think of molts in terms of different plumages.

Young eagles go through four different plumages until they reach their sexually mature, adult plumage, which would be the fifth plumage type. These are (as described by Clark and Wheeler in Hawks of North America):
Juvenile, White-belly I, White-belly II, and Adult transition plumages.

So, you might think, ok, 5 years to sexual maturity, 5 plumages, one molt per year, right? Not exactly. Molt can be affected by a variety of biological and welfare factors (such as food supply, density of other eagles, and others), and not all molts are always complete molts.

Once they achieve their final “adult” plumage, it is likely that bald eagles molt their flight feathers just about every year, primarily in New York from summer through fall. However, some evidence of molting can be seen at almost any time of the year.
This flight feather molt is not simultaneous; rather, matched flight feathers are generally lost at separate times, so the birds are never left flightless.

Q. How many muscles do eagles have?
A. A very interesting question that sent me to the books! I found no specific reference to bald eagles, but the Audubon Encyclopedia of North American Birds says that 175 different muscles, most of which are paired, have been described in birds. The number in eagles is likely close to that. The legs alone contain about 35. Muscles make up somewhere between 35 to 60 percent of the total mass (weight) of birds.

Q: How much can a Bald Eagle lift?
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 now 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 more 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, and the eagle is very capable of floating and "swimming" to shore with its prey, rather than give it up. Then, dragged up on shore, the feasting begins.

Q. How can eagles see the fish under the water?
A. Eagles have very good eyesight, 3-4 times better than you or me. They can see fish a long way away, including down in the water a ways. Mostly though, the fish eagles are capturing are very near the surface of the water.

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!

Q: Do eagles see in color or black and white?
A: Great question! Everyone knows bald eagles have superior eyesight. We believe they can see in color based upon the more numerous cones in their retina. Cones are known to be necessary for acuity and color vision, versus the rods which are for sight in low-light conditions, something eagles are not especially adapted to.

Q: Do eagles see in black or white or color? Do they dislike the color red?
A: I knew I had seen a very thorough treatment of eyes and sight somewhere, so below is what I found at this web-site: "All eagles are renowned for their excellent eyesight, and the bald eagle is no exception. They have two foveae, or centers of focus, that allow the birds to see both forward and to the side at the same time. Bald eagles are capable of seeing fish in the water from several hundred feet above, while soaring, gliding, or in flapping flight. This is quite an extraordinary feat, since most fish are counter-shaded, meaning they are darker on top and thus harder to see from above. Fishermen can confirm how difficult it is to see a fish just beneath the surface of the water from only a short distance away. Young bald eagles have been known to make mistakes, such as attacking objects like plastic bottles floating on or just below the surface of the water. Bald eagles will locate and catch dead fish much more rapidly and efficiently than live fish, because dead fish float with their light underside up, making them easier to see. Eagles have eyelids that close during sleep. For blinking, they also have an inner eyelid called a nictitating membrane. Every three or four seconds, the nictitating membrane slides across the eye from front to back, wiping dirt and dust from the cornea. Because the membrane is translucent, the eagle can see even while it is over the eye. Eagles, like all birds, have color vision. An eagle's eye is almost as large as a human's, but its sharpness is at least four times that of a person with perfect vision. The eagle can probably identify a rabbit moving almost a mile away. That means that an eagle flying at an altitude of 1000 feet over open country could spot prey over an area of almost 3 square miles from a fixed position."

Q: How long are a bald eagle's talons?
A: Eagles have 4 talons (and toes) on each foot, a hallux talon at the back of the foot that faces front, and 3 toes on the front of the foot where the talons face toward the back. The hallux talon is always longer than the other regular talons. And in females, this talon is longer than in males. As a matter of fact, that is one way we tell the gender of bald eagles, by measuring the hallux talon, as the females is longer than the males. These hallux talons are almost 2 inches long on large, female eagles, and only about an inch and a quarter on small males.

Q: Are talons made of the same thing as human fingernails?
A: Yes, and they are very similar to a dog's nails.

Q: Are the talons strong?
The real strength comes from the muscles in the legs, that when contracted clamp the tendons in the lower legs and toes down, closing all the talons together in a vice-like grip. I remember this grip well, once getting fully footed by an eagle we captured; it took two of us to pry the toes apart and extract the talons.

Q. Do bald eagles have the same digestive system as humans?
A. Now there's an interesting question! The answer is, no. Birds in general have a higher metabolic rate than we do, which demands that they process their food as quickly as possible. This means getting it into a form from which they can extract the energy they need, quickly and efficiently. Birds, including eagles, have adaptations for doing this. Most importantly, part of their stomach has turned into a gizzard, which we don't have, in which food is ground down to a fine consistency to permit rapid digestion. In eagles, this is also the place where pellets are formed. These are masses of material from prey that cannot be digested, such as fur, feathers, and occasionally bone, that then travel backwards from the gizzard up to the mouth and are cast (like vomited) out the mouth. Depending on what they have eaten, pellets are formed after the meal, overnight, and are usually cast out the next morning. Most fish are digested completely. Eagles have very strong stomach acids, and can digest bone quite well, which aids them in their own bone formation and in their egg-shell formation. Another major difference is that eagles (and other birds) have something called a crop, in the upper alimentary track (esophagus) where food can be stored for days. This is extremely beneficial to eagles, who can store up to two pounds of food in their crop when prey is abundant, so they can then go without food for several days if need be. There are more differences, but these are two of the major ones.

Q. Do eagles sing?
When canoeing on Upper Klamath Lake, Oregon, last August, we saw an adult bald eagle coaxing an adolescent to fly. Finally, the young eagle flew from the nest. When the adult returned, after circling a number of times and followed shortly by the younger bird, one of them burst into song. We've never read about eagles singing (other then their normal screech). Is this common or a fluke of nature?
A. Lucky you, being up on Klamath Lake! Adults will coax their young to leave the nest, just as you observed. Eagles vocalize regularly. They have only a few types of vocalizations, and virtually nothing is really known of their meaning (although many guess at them). Many people also speak with disrespect about the "whimpyness" of eagle calls, but I guess it depends on your perspective. I have always found their primary call, one I describe as a descending chitter-call, kind of like a laugh, as pretty eerie and penetrating. What they are trying to say, I don't know, but they use this call under many different circumstances, and the one you describe does not surprise me in the least. They often use this call when other eagles are present, or are flying in, say to a feeding area or roost or perch. It could be a warning, or a greeting, or? One of the things I would like to know some day, is what eagles are saying to each other!

Q: Do the eagles have a special technique for breathing when they fly so high?
A: Good question! This sent me to the bird physiology books myself! In reality, eagles tend to use very little energy when they fly so high. Even though they can reach altitudes of over 10,000 feet, they are usually soaring to these heights, and taking long glides to cover ground, then soaring up again and repeating the process. By flying in this way, their body is really not demanding much oxygen, not anywhere near as much as when they are much closer to the ground and expending considerable energy flapping their wings. So, no, they have no special adaptations for breathing at high altitudes.

Q: Does an eagle fly aerodynamically like an airplane?
A: All birds share aerodynamics of airplanes (actually it is airplanes that have "learned" their aerodynamic lessons from birds!). Bird wings are designed to achieve lift through differences in pressure as air moves over and under the wing. (See "Bernoulli's principle of flight.") Birds, including eagles, also have hollow (yet very strong) bones to decrease total body weight and help in flight. Unlike many other birds, though, eagles are fairly heavy, bulky birds. Because of this, they need a lot of room and a lot of energy to take off, not unlike an airplane. They are not birds designed for quick flight and maneuvers. Rather, they really shine as soaring birds, using their large wings, once airborne, taking advantage of air currents and natural thermals, to float almost effortlessly at great heights and over long distances.


Peter E. Nye
New York State Dept. Environmental Conservation
Division of Fish, Wildlife and Marine Resources
Albany, NY