Bald Eagle Facts
Q&A with Peter Nye in 1997
New York Department of Environmental Conservation
Characteristics
Q. How many feathers in the average adult eagle?
A. 7,200.
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. 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, the 4th (the hallux) faces backward to aid in gripping prey.
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. 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.
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.
Life Cycle
Q. What is an eagle's average lifespan?
A. Probably between 20-30 years.
Q. What age do the Eagles reach their sexual maturity?
A. Five years old, but they can breed as early as 4, although the full white head and tail is not obtained until 5-6 years of age.
Q. How do Bald Eagles mate? Do they have any mating
displays?
A. The male mounts the back of the female, literally standing on her back while on a limb or the nest. They do have courtship displays involving aerial acrobatics and tumbling.
Q. How many eaglets do female eagles have in a lifetime?
A. Assuming a reproductive life span of 25 years - and an average of 1 young/year - she would have 25 eaglets.
Q. Are Bald Eagles monogamous?
A. Yes, they only have one mate at a time.
A. Yes.
Q. If eagles mate for life, what would happen if one died?
A. The remaining bird finds a new mate and usually carries on in the same nesting territory.
Q. Do bald eagles establish a territory? If so, how?
A. Yes. They select a breeding territory based upon an evaluation of all the things they are looking for: food supply, suitable nesting and perching trees, and isolation from excessive human activity. These areas are usually near where they themselves fledged as youngsters. Wintering birds do not establish a territory, but usually do use the same area each winter.
Q. What are the size and characteristics of the Bald Eagle's egg?
A. Bald eagle eggs are unremarkable in appearance; dull white with occasional pale brown sploches, but not usually. They are shaped like most eggs, and weigh around 120-130 grams 28 grams/ounce). They range from 6-8 cm long (2.54 cm/inch) and 5-6 cm wide.
Q. If the baby is late being born, how does it migrate?
A. No matter when they are born, they migrate the same and always have time.
Q. Why are baby eagle's heads brown while the parent's heads are white?
A. The overall brown and white mottling of young eagles before they reach sexual maturiy is likely an adaptation to keep them more concealed and thus protected while they are young.
Ecology
Q. What is the eagle's favorite food?
A. Fish
Q. What are two things eagles like to eat a lot?
A. Fish and birds.
Q. What is the average amount of fish an eagle can eat in a day?
A. They eat 5-10% of eagles' body weight.
Q. How much and what food do they eat?
A. Diet varies with season and with what is available (eagles are very opportunistic in their feeding habits), but fish is the number one food type followed by some birds (waterfowl), occasional turtles and other rarer items. Average daily food consumption is from 250-550 grams per day, or between 5-10% of an eagle's body weight.
Q. What are the Bald Eagles' natural predators?
A. Besides humans? Very few. Other bald eagles occasionly attack eagles and their young; racoons will sometimes kill young in the nest as will the occasional great horned owl.
Q. What is the lowest and highest temperature eagles can live in?
A. Another great question! Bald eagles live in dramatic temperature extremes, from hot deserts of Arizona and south to near the treeline in extreme northern Canada and Alaska. They seem quite adaptable to weather/temperature extremes.
Q. Is migration of the bald eagle based on availability of food source, temperature change, or what?
A. Mostly based upon available food supply, right on!
Q. How does the freezing weather affect the Bald Eagle?
A. It freezes up their water areas and forces them (usually south) to areas of milder weather for open water.
Q. What is the Bald Eagle migratory pattern?
A. Eagles aren't really migratory like some birds, but move only as far as they need to to survive (to find open water and food).
A. Northern eagles typically move south to where they can find open water and food.
Q. Do Bald Eagles go back to the nest where they were born or do they go to a different nest?
A. They do not go back to the exact nest where they were born (hopefully their parents would still be nesting there), but there is a concept called "site-fidelity" which means there is a strong attraction for a bird to return to the area where it was born. This "area" varies in size depending upon many factors such as available habitat and the existing density of eagles (i.e. is there room for more?). We have followed many young eagles who have subsequently returned to from 1-225 miles from where they were born to start their own nests.
Q. How do the eagles find their way home?
A. If you mean home as in where they fledged from the nest, a variety of cues are used including mostly physical landmarks.
Q. We live in Florida where the eagles don't migrate, so how far north are eagles where they don't migrate?
A. Some eagles way up even in Alaska don't need to migrate or
move because they can stay near the open ocean all winter and still get
food. Believe it or not, some of your Florida eagles do migrate north
following their breeding season; they have been recorded here in New York
and even in eastern Canada. Why do they do this? Not sure, perhaps to
escape the intense heat of summer. It is common for Florida fledglings
to move north, but most of your adults do not.
Q. How do the transmitters (for satellie tracking)
work?
A. They carry a battery and emit a specific radio signal picked up each time two orbiting satellites (12,500 miles above the earth) pass over their location. The satellites read the message sent by the transmitter and calculate its location (the latitude and longitude). The battery lasts for approximately 1-year. The satellite then sends this data back to earth to computers in Maryland, and I access those computers and get all the information. Then you guys get it - pretty cool huh?
Conservation
Q. What is the Bald Eagle's biggest enemy?
A. Humans.
Q. What were eagles killed for?
A. Eagles used to be killed for many reasons; for fun, for mounted trophies, and because people mistakenly thought eagles were taking too many fish and competing with human fishermen.
Q. What is the population of Bald Eagles in the United States.
A. As of Spring 1997, about 5000 nesting pairs in the continental U.S., about 14000 birds in winter. In Alaska, there are around 25,000 bald eagles.
Q. When you take the blood sample of eagles, what are you looking for?
A. Organochlorine contaminants such as PCB's and DDT and metals such as lead, mercury and cadmium. The levels we find may indicate that these birds are eating contaminated prey and may have reproductive problems.
Q. Do you think the Bald Eagle population will recover? By how much?
A. Absolutely! As of 1997, it has already been recovering for the past 10-15 years, largely in response to elimination of DDT from its environment; eagles are now breeding successfully and producing many chicks. The key now and in the future to the bald eagles survival (and to all plants and animals for that matter) will be to ensure that they have enough space (habitat) to fulfill their life functions. Humans must set aside enough land for eagles and the ecosystems which they depend upon to survive if we want to have them around in another 100 years for all of your grandchildren. Within the continental United States we currently have about 5000 nesting pairs; I would suspect we could get to 10,000 on the top end.
Q. In the past 10 years (since 1986), how has the population changed and have the causes of death changed?
A. Many more nesting pairs have been established over the past 10 years, thanks to active restoration programs in many states and to increased fledgings and survival. In NY we've seen about a 200% increase over the past 10 years; throughout the lower 48 states, the nesting poulation has probably increased about 20-30% in that time. Causes of death remain the same, almost all linked to human beings - habitat loss and alteration are the most important factors today.
Q. Why is the Bald Eagle our national bird?
A. It was selected as such by our continental congress in 1782 after considerable debate on the issue; Ben Franklin suggested the wild turkey might be an appropriate choice. I suspect that the beauty, power, grace and spirit of the eagle were qualities we wanted to adopt in our symbol.
Q. What caused the death of so many Bald Eagles last year (1996) in the U.S. during their winter migration?
A. This question is still being investigated, but evidence seems to be pointing to some sort of chemical contaminant or compound used to kill or treat other animals. I just received the quarterly report from the National Wildlife Health Center (NWHC) for the period from October through December 1996. It had the following on the bald eagle mortality event in Arkansas:
"A large scale bald eagle mortality event has occurred for the second time in three years in southwestern Arkansas. In early November, bald eagles and American coots were found sick and dead on De Gray Lake. From November through January, a total of 25 affected eagles were found on Lakes De Gray, Ouachita and Hamilton. In general, the birds were in good body condition with no gross lesions. All ages and both sexes were affected. The only abnormality noted in eagles and affected coots were microscopic changes in the white matter of the brain, described by NWHC pathologists and others as vacuolar myelinopathy. This is the same lesion noted in the 1994/1995 mortality event. Extensive diagnostic testing has ruled out bacterial, viral and parasitic diseases. It is most likely that the mortality is caused by a toxin, either manmade or naturally occurring. Comprehensive toxicology testing has not, as yet, revealed a cause for the deaths. This mortality event has been the focus of intensive diagnostic, research and field study. NWHC has been working with a multi agency task force on disease identification, management and prevention."
Other Questions
Q. Does it hurt the birds when they carry around that transmitter?
A. We don't think so, but it is hard to know what an eagle is thinking! They seem to be able to carry it fine and go about their normal day-to-day business without any effect. The transmitters are designed to fall off the birds after a few years. You raise a good point though, and one which wildife researchers constantly need to ask themselves; is the impact on my study animal worth the information?
Q. Why are you so interested in eagles?
A. Why not? Somebody has to be! It is part of my job as a wildlife biologist for the New York State conservation department responsible for rare and endangered wildlife (I also love it :)
Q. Why did you choose bald eagle if butterflies were your favorite animal?
A. I didn't really "choose" bald eagles, as I said in answer to another question, working with eagles is a part of my job as an endangered species biologist; I also get to work with rare butterflies and turtles, snakes, bats, dragonflies and all sorts of neat critters!!
Q. Have you studyed other birds besides the Bald Eagle?
A. Yes, many, including peregrine falcons, ospreys, kestrels, loggerhead shrikes, and several others. As a matter of fact, we have a similar satellite tracking project involving ospreys, which we found migrate to South America.
Peter
E. Nye
New York State Dept. Environmental
Conservation
Division of Fish, Wildlife and Marine Resources
Delmar, NY
Spring, 1997
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