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Height: 28 - 38 inches Wingspan: 78 - 84 inches
Sexes look alike, but male is usually smaller. The characteristic white head and tail do not appear until the birds reach maturity (4 - 6 years).
Adult bald eagles generally mate for life, but if one of the pair leaves or is killed, the survivor will take another mate. However, the nest might be abandoned, especially if it is the female that is lost. Whether the birds are taking their first mate, second, or third, the process is the same: pair-bonding (which can continue throughout the year), nest-building, and production of young - a task that will be repeated year after year until the pair is more than 20 years old.
Pair-bonding in bald eagles involves several things - most notably of which are the aerial displays that accompany the ritual. One bird will dive on another and, at the last second, the bird being "attacked" will turn over on its back and extend its talons into the air. Contact between the two birds is rare. Even as dramatic as these displays may be, they are usually short-lived and infrequent. And it is not always adult birds seen doing this. In fact, it is usually two immature birds, or an adult and a subadult bird. The extent of most displays is generally just two birds soaring together or chasing each other.
In chases, one bird will chase after the other, usually in a rather shallow glide. It is also not uncommon to see them switch places during chases. They might also be seen passing sticks in mid-air. But the most thrilling display of all is when they actually lock talons in mid-air and tumble down or swirl around and around as they fall, breaking apart when they sometimes seem to be dangerously close to the ground.
The final sight that would signify pair-bonding is side-by-side perching. This usually happens in the near vicinity of the nest, sometimes on the nest itself. They may even lie in the nest right next to each other.
Copulation usually occurs (no matter what Hollywood might portray) at the nest. The male will simply mount the female to make contact. The whole thing lasts just seconds. After copulation the pair might perch next to each other for a half hour or so, sometimes preening themselves and each other. They also might be seen rearranging nest material during this time.
A bald eagle’s nest grows with each year of use. They usually start with one of the taller trees in a given area, with a network of strong supporting branches. While the nest may start out only a couple of feet wide, after a few years of use, the nest can grow to more than six feet wide and ten feet deep! The male usually brings nest material to the nest site where the female will arrange it to suit her. Occasionally, the male will be seen passing sticks to the female on a branch near the nest or, on even rarer occasions, the sticks may pass between them in mid-air. Softer material, such as grasses and leaves, will be used to line the center of the nest.
Bald eagles generally lay two eggs, although one or three are not that uncommon. There have even been cases of up to four eggs in a nest, but many researchers believe that this might be due to polygamy on the part of the male – eggs from two different females will be incubated in the same nest. The eggs are laid about two days apart and will normally hatch in the same order as they were laid, with approximately the same intervals between hatchings as there were between layings. The eggs are bluish-white, about 3 inches in length, and roughly oval-shaped. Through time, the eggs will discolor until they appear to be more of a mottled, or dirty, white. Incubation lasts 34 to 36 days with both the male and female birds incubating, although the time devoted to incubation is not evenly split between the pair. Females will incubate the eggs about 60 percent of the time. A typical stint on the eggs lasts anywhere from 30 minutes to two or even three hours. During incubation, the male will bring food for the female, many times to one of the supporting branches of the nest, but she will usually come off the eggs to eat, with the male taking her place on the nest. On other occasions, the female may hunt for herself while the male takes his turn on the eggs.
For the most part, bald eagles eat fish, at least during the warmer months of the year when open water prevails. Hunting from a perch, bald eagles can exhibit patience that humans only wish they had. An eagle might sit for hours in one spot and when a fish comes too close to the surface, they dive at the target at a medium angle, extend their talons in front of their body, and snatch the fish from the surface of the water. The same attack process is used when lakes, rivers, and streams start to freeze over, leaving small openings in the ice cover, only now their target is more likely to be waterfowl. Ducks and geese tend to sit on the edge of the ice next to these openings and go into the water to escape the talons of predators such as the bald eagle. Eagles use the same snatching techniques to grab their prey whether the prey item is on the ice or in the water. When bodies of water freeze over completely, most eagles will migrate to the nearest area with open water. But, at the same time, their menu preferences will switch to whatever food is most available, especially carrion. One winter bald eagle census technique involves the placing of road-kill animals as bait to draw the eagles into an area for an accurate count.
The first ninety days of life are considered the nestling stage for bald eagles, although they may fledge in anywhere from 72 days to as much as 98 days. It is during this time that they will acquire some 95% of their adult size - growing from a hatch weight of some 10 ounces to their adult weight that might reach 12 pounds. During the nestling stage of an eagle’s life, its very survival might depend on how many siblings (or half-siblings) are in the nest and in which order they hatched. An eaglet that is last to hatch in a brood of three has little chance of survival. First of all, the siblings will all be bigger and, therefore, out-compete the little one for food. Secondly, older siblings have been known to shove their way around the nest, sometimes inadvertently killing the youngest or even pushing them out of the nest. And finally, the larger siblings in a nest have been documented on several occasions, killing and eating the youngest of the brood. If all three (or even four) siblings survive, it is probably because the parents are on an exceptional piece of habitat, where feeding themselves and three or four young is possible without causing undue strife at the nest between the youngsters.
Should the young survive to about 12 weeks of age, the time has come for them to try their hand at this thing called flying. Contrary to the more anthropocentric view of birds, adult eagles (or adult birds of any kind) do not teach the young how to fly – no more than humans teach their youngsters to walk or run or jump. It is a part of their inherent behavior and when the right time comes, assuming they are physically capable, the young will take off from the nest. The first flight is generally a short one and, typically, to a perch lower than the nest site – it sometimes seems more of a gentle glide than actual flight, but the landing might be anything but gentle. It takes a little practice to figure out when to stop flapping their wings while grabbing a perch, and the result can, surprisingly often, be a perfect two-point landing, except that the bird ends up upside-down. Injuries rarely occur during these episodes and in just a couple more attempts, the young eagles will look like they’ve been landing on rickety branches for years.
At this time, it might be possible to distinguish sexes amongst the youngsters. Females are generally larger than males, but there is some overlap between the sexes, making sex determination of eagles in the wild somewhat difficult. Females range from about 9 to 11 pounds at maturity, while males range from about 8 to 10 pounds. Therefore, a larger male and a smaller female will appear to be exactly alike. There are no other phenotypical traits that can be used to determine the sex of bald eagles in the wild.
For the first month or so after fledging, the nest site is still the focal point of a young eagle’s wanderings. They will usually stay within a half of a mile or so from the site. During this time, one or more of the parent birds might be seen feeding the youngster at a perch in the immediate area. But, as the young bird matures more and more, its daily forays will spread to points farther and farther from the nest. It is during this critical period that an eagle forms its idea of “home.” This tendency to return to a “home area” is what has made the various reintroduction programs of bald eagles so successful all across their range.
Eagles hatched in the more northern climes of the continent will have to move in the fall as the lakes and rivers where they fish freeze over for the winter. But, unlike other birds that migrate to a specific location somewhere to the south, eagles seem to only travel as far as needed to find open water – an area with a readily available food source. This is one of the reasons that Lake Monroe is a popular winter destination for eagles from up north. Lake Monroe generally doesn’t freeze completely over during the winter months. And even in the colder years, ice-eaters at the Fourwinds Resort and Marina keep open water available for eagles.
While eagles from other areas can readily be found at the lake during the winter months, the resident pairs of the lake don’t seem to move anywhere at all. If they do make a winter trek, it probably amounts to no more than an occasional feeding foray to the Ohio River area. It might be thought of as more of a winter dispersion than a seasonal migration. The result is Lake Monroe’s eagle population grows each winter as the resident birds are joined by others migrating from the frozen lakes, streams, and rivers to the north.
For the next four to six years, an eagle’s life is spent hunting and eating. During this time they will probably start forming some sort of a bond with another eagle, and nest-building might even begin, although the eagles will not be able to produce until they are about five years old, the time when they undergo their final plumage change and sport the distinctive white head and tail.
National Audubon Society's The Sibley Guide to Bird Life and Behavior, Alfred A Knopf, New York, 2001.
National Geographic's Field Guide to the Birds of North America, National Geographic Society, Washington, D. C. 2001.
Stokes Nature Guides A Guide to Bird Behavior, Volume III, Donald and Lillian Stokes, Little, Brown, and Company, Boston, 1989.
The Birder's Handbook - A Field Guide to the Natural History of North American Birds, Paul Ehrlich, David S. Dobkin, and Darryl Wheye, Simon and Shuster, New York, 1988.
© Copyright 2004 Jeff Riegel
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Bald eagle portrait image by Hammond Photography - Jeff Hammond, Photographer; Laura Hammond, Business Manager. Other photos courtesy of Indiana Department of Natural Resources.
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