Bald Eagle Biology
How many species of eagles are there?
59 species worldwide. Birdlife.org lists all the species with links to more information.
How large is a bald eagle?
Northern eagles are larger than Southern eagles. Male Bald eagles’ weight may range from 6 to 9 pounds, with females’ weights usually 20 to 30 percent greater. Alaskan females reach up to 15 pounds. Florida males may weigh only 6 pounds. The average female Bald Eagle is 35 to 38 inches.
The wingspan varies from 6 to 8 feet.
What are the differences between bald and golden eagles?
The primary difference is that Bald Eagles belong to a group of “sea” eagles that 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 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 its name from the old English word “balde” which means white-headed (not hairless!). “Golden” eagles likely got their name from the top and back of their head and neck, which are a beautiful golden color. – PN
How do bald eagles control their body temperatures?
Eagles adapt to the changes in temperature very efficiently. They have an under layer of fluffy down feathers under their outer feathers to insulate them from the cold. They “thermoregulate” (control their temperature) by panting with their mouth open or through heat loss through their unfeathered legs and feet. Babies are able to “thermoregulate” when they reach the age of 10 – 14 days old. Until then, the adult parents (usually the mother) sticks closely to the babies so they do not become too cold.
What are the differences between male & female bald eagles?
Females on the average are about 1/3 larger than males.
Two 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.
The fluting calls of males is almost a scream; females is pitched much lower. – PN
What is the lifespan of a bald eagle, & how long can they reproduce?
The life span of eagles in the wild is generally around 30 years. The longest that any Bald Eagle has been known to live in the wild is 39 years. In captivity, they may live over 50 years due to fewer hazards and veterinary care.
About 50 percent died during the first year due to their inexperience at meeting the dangers of living in the wild. After their first year, about 90 percent survive each year.
Eagles are thought to be able to reproduce throughout their lifespan, but little documentation is available. One eagle has been documented successfully raising young in her 26th year.
What are some facts about bald eagle feathers?
Bald Eagles have 7,200 feathers. To see examples of the feathers, click here.
The feathers of a bird are superbly crafted to form its aerodynamic shape and protect it from the challenges of water and weather. In this sequence from FLIGHT: THE GENIUS OF BIRDS slow motion photography and computer animation showcase remarkable levels of engineering and design.
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. The skin surrounds and grows over the shaft.
A pin feather, sometimes called a “blood feather,” is a feather that is developing on a bird. It has a blood supply flowing through it, and if it is damaged, a bird can bleed heavily. As it grows longer, the blood supply is concentrated in only the base of the shaft. At this point, it is no longer called a “blood feather.” The feather comes out wrapped in a thin shaft of tissue, which will eventually split, which allows to unfurl and grow to its full size.
Eagles go through a molting experience with their feathers. 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, 5 years to sexual maturity, 5 plumages, one molt per year. 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. 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. – PN
What special muscles help with flight?
A network of hundreds of muscles, ligaments, and tendons are required for flight. In this sequence from FLIGHT: THE GENIUS OF BIRDS the structure and design of a snow goose’s primary flight muscles (the engines that elevate and drive it through the air) are demonstrated in stunning detail.
What is a brood patch?
A brood patch is an area on the parent’s chest that does not have feathers. This is the area that touches the eggs while the parent is incubating them, and allows for a more efficient transfer of heat to the eggs.
Not all birds develop a brood patch. In species as the bald eagle, both parents develop an incubation patch because, as we see each day, they both share the incubation duties. The incubation patch begins to develop on the breast or abdomen shortly before the female lays her eggs through hormonal changes that cause the feathers that cover that area to fall out on their own. That leaves a wrinkled patch of bare skin that blood vessels fill with warm blood. When we see the female or male “wiggle” as they settle upon the eggs, they are spreading that bare patch over the eggs to keep them warm. ….(Courtesy CCB Nest Blog)
What’s happening inside those eggs?
In this sequence from Illustra Media’s newest documentary FLIGHT: THE GENIUS OF BIRDS you will enter a fertilized egg to witness a bird’s embryonic development. Spectacular animation and live action footage document the extraordinary 21-day process of organization and growth from a few cells into a chicken.
In a Bald Eagle, approximately 35 days are required for the embryo to develop into a fully-developed eaglet once incubation begins.
Why do eagles turn the eggs while they are incubating them?
The eggs are rolled over by either parent about every hour to 2 hours during the incubation period. The purpose of this roll is to make sure that the lighter yolk does not rise to the egg surface and the delicate blood vessels that cover the yolk touch and stick to the shell surface, killing the developing chick. (Peter Nye)
Raptor Resource (Bob Hancock) adds: Turning or rolling assists air exchange, helps maintain an even egg temperature.
Why do eagles sometimes leave the eggs uncovered during incubation?
In response to a question from a Decorah cam viewer as to why the Decorah adult eagles stayed off their eggs during a particular period of time, Raptor Resource explained: “A bird’s eggshell has thousands of tiny pores, which allow water and gas to pass through. Mammals like us get oxygen through an umbilicus, but developing birds receive oxygen and remove carbon dioxide through the egg shell. Gases, including oxygen, enter and leave the egg by diffusing through the pores in its shell, across the outer and inner shell membranes, and into the blood in the capillaries of a special tissue called the CAM, or chorioallantoic membrane. As the weather warmed in Decorah, the snow began to melt and the humidity soared. Condensation can form on eggshells exposed to excessive humidity, which clogs shell pores and provides a vehicle for bacteria. The result? Fatal suffocation and/or contamination. Only the eagles know for sure, but I think they may have responded to the threat of rising humidity levels by leaving their eggs uncovered. Standing or leaving entirely allows fresh air to circulate over the eggs, dropping the humidity level and giving the developing embryos fresh air.”
This was an explanation to a specific situation – but the overarching reason seems to be rising humidity, clogging shell pores and thus providing a vehicle for bacteria.
How does an eaglet hatch?
After approximately 35 days, the eaglet inside the egg is ready to hatch. An “egg tooth” has been formed at the top of its beak to assist in this process. In addition, a very strong muscle on the back of its neck has developed, which assists with the work of punching through the membrane inside the egg.
Inside the egg, at the top, is an air bubble, and breaking the membrane inside the egg allows the eaglet to breath its first breath of air inside the egg shell. This little bit of air will give the eaglet energy to continue to break free of the shell. During this time, the yolk of the egg is absorbed into the stomach of the eaglet, providing more energy.
Using its egg tooth, the eaglet scratches around the inside of the shell to weaken it. Finally, a tiny hole or crack emerges. This is called a “pip.” Breaking free from the egg is an extremely tiring process for the eaglet, and can take up to 2 days from the first pip to an actual hatch. During the process, the eaglet sometimes will rest for awhile. During the hatching process, the eaglet slowly rotates counterclockwise by pivoting its legs, all the while scratching the inside of the shell with its egg tooth.
A hatch is complete when the eaglet is totally out of the shell.
The parent eagles do not assist in the process, but they seem to be aware when the eaglet is ready to hatch. They can even hear the tiny peeps coming from inside the shell, and will occasionally move off the eggs they are incubating and look down to see what is happening.
In a post from 2018, “Elfruler” (www.elfruler.com) states, “The avian egg is a marvel of nature, a self-enclosed and perfectly effective living environment for the developing bird embryo. It is sturdy but flexible, hard but porous. It contains all that is necessary to enable a small and weak organism to develop into a chick with enough strength and skill to break through and emerge into the outside world. Click here for an account of the many factors involved in a chick’s hatching.” (Extremely detailed and more suitable for older students and/or adults)
*Note: “Elfruler” is the nomme de plume of the author of a very well known Bald Eagle Blog (www.elfruler.com). “Elfruler” is a “retired university professor with a Ph.D. in a discipline in the humanities, has been a birdwatcher for many years and has been an avid observer of internet bird cams (raptors and other wildlife) since 2009. During that time she has also read widely and deeply in ornithology, and for 4 years she has volunteered for raptor rehabilitation organizations, gaining hands-on experience in the capture and rescue, medical triage and treatment, and rehabilitation of raptors and other birds.”
How do baby eaglets grow?
Baby eaglets come into the world totally helpless. They cannot hold their head up; their vision is limited; their legs are too weak to hold their weight. Bald eagles are altricial, which means they must rely 100 percent on their parents to protect them and care for them.
It can take days for them to completely hatch from the first pip to being totally free from the shell (in the nest of Romeo & Juliet in Florida, the first eaglet hatched (NE16) in 2016 took 40 hours to complete the process. Often, it’s much quicker than this.
After hatching, the eaglet will dry off and fluff up to a downy gray. Food will be offered to the eaglet by the parent, who shreds meat off fish or whatever is available. Tiny pieces will be offered again and again as the eaglet struggles to hold his wobbly head still long enough to take the food. In a short time, the eaglet becomes stronger and his eating skills and coordination develop quickly.
An eaglet has a crop – a storage area – below its chin. Food goes into the crop and is then digested as needed. When the crop is “full” you can see it bulging out. This crop is actually part of the esophagus where food is stored and softened. The crop regulates the flow of food through the digestive tract.
How quickly do the eaglets grow?
- The eaglets grow rapidly, they add about a half pound to a pound of body weight every week until they are about 9-10 weeks old, depending on if the eaglet is a male or female. Females are always larger.
- At about two weeks, it is possible for them to hold their head up for feeding. At this age, the eaglets can also thermoregulate. Thermoregulation means that the eaglets now have the ability to maintain a near constant body temperature. They don’t have this ability at birth, so the parent eagles must brood them consistently until the eaglets reach this important milestone.
- At about three weeks they are 1 foot high and their feet and beaks are very nearly adult size.
- At about three to four weeks old the eaglets are covered in a secondary coat of gray down.
- At about four to six weeks, the birds are able to stand, at which time they can began tearing up their own food.
- At about three to six weeks, black juvenile feathers will begin to grow in. While downy feathers are excellent insulators, they are useless and must be replaced with juvenile feathers before an eaglet can take its first flight, some 10 to 14 weeks after hatching.
- At about six weeks, the eaglets are very nearly as large as their parents.
- At about eight weeks, the appetites of the eaglets is at its greatest. The parents will hunt almost continuous to feed them, meanwhile at the nest the eaglets are beginning to stretch their wings in response to gusts of wind and they may even hover for short periods. The eaglets grow stronger.
- At about nine to ten weeks, they begin branching, this is a precursor to fledging.
- Around ten to fourteen weeks, the eaglets will fledge, or fly away from, the nest.
- Once the eaglets have fledged they may remain around the nest for four or five weeks, taking short flights while their primary feathers grow and strengthen. Their parents will still provide all of their food. The juvenile fledglings, with the exception of their color, look similar to their parents, but are nothing like them in behavior. The juveniles now have to learn to hunt, and they only what’s left of summer to learn. After that, they’re on their own. The first winter is the most dangerous and difficult part of an eagle’s life.
Does an eagle have exceptional eyesight?
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.
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. 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 visions, versus the “rods” which are for sight in low-light conditions, something eagles are not especially adapted to.
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.
A Bald Eagle’s eye changes color as it reaches maturity. 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.
What special features are found on bald eagles’ feet and talons?
The feet are featherless (scaled). They have 4 toes, each with a very serious claw (talon). Three toes face forward; the 4th (the hallux) is longer and faces backward to aid in gripping prey. These hallux talons are almost 2 inches long on large female eagles, and only about an inch and a quarter on small males. Talons are made from the same material as human fingernails, and are very similar to a dog’s nails. The real strength of talons comes from the muscles in the legs. When they contract, they clamp the tendons in the lower legs and does down, closing all the talons together in a vice-like grip. – PN
What are the features of a bald eagle’s digestive system?
Birds in general have a higher metabolic rate than humans, 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, 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 feature in their digestive system 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. – PN
Visual description of Bald Eagle’s beak & eye.