An online resource based on the award-winning nature guide

Bald Eagle

Bald Eagle Eyesight

7-26-16  juv. bald eagle2 026The eyesight of a Bald Eagle is impressive.  Part of the reason for their excellent vision is that these birds of prey have two centers of focus (foveae), which allow them to see both forward and to the side at the same time. Cone cells, one of three types of photoreceptor cells in the retina, perceive color, fine detail and rapid movement.  In a human, the fovea has 200,000 cones per millimeter; in the central fovea of a Bald Eagle’s eye, there are about a million cones per millimeter. 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.

Bald eagles are capable of seeing fish in the water from several hundred feet above, while soaring, gliding or in flapping flight.  (They 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.)  It is very likely that a Bald Eagle can identify a rabbit moving almost a mile away. This would mean 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. (photo: recently fledged, juvenile Bald Eagle)

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Sea Lampreys Spawning In Connecticut River

e-eagle with sea lamprey 325Sea Lampreys aren’t a common sight, but an opportunity presented itself after this Bald Eagle “rowed” from the middle of the Connecticut River to the shore with its too-heavy-to-lift prey, ate a portion of it and flew to its nest with a slightly lighter load.  At first glance I thought it had captured a large snake, but closer inspection revealed that a three-to-five pound Sea Lamprey approaching 30 inches in length was clasped in the eagle’s talons!

Lampreys were accidentally introduced into the Great Lakes and Vermont’s Lake Champlain, where they became parasites of other fish due to being landlocked.  However, during the time the native Sea Lampreys are found in the Connecticut River, they are not parasitic. These fish are anadromous – they live as adults in the ocean (where they are parasitic) and return in May and June to spawn in fresh water.

Sea lamprey young spend three or four years as worm-like creatures burrowed in the soft mud of the Connecticut River.  When they reach five or six inches (which can take up to ten years), young lamprey head for the sea. The ocean-dwelling adults uses their round, rasping mouth – filled with concentric circles of teeth – to scrape a hole in the side of a host fish and feed on blood and body fluids before letting go. They weaken, but don’t kill, their hosts. After spending one to two years in salt water, Sea Lamprey head back to the closest freshwater stream or river, migrate upstream, cease feeding and spawn.  They never return to the ocean, as they die after mating and laying eggs.

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Bald Eagles Tending Young Eaglets

5-2 eagle and chicks 831

In the Northeast, Bald Eagle eggs are hatching and the heads of the one-to-three chicks can be seen bobbing up and down, anxiously begging for a tidbit of food from one of their parents.  For the first two or three weeks, their mother stays with them 90 percent of the time, keeping them warm and tearing food brought by their father into little pieces that she feeds to her chicks.  Eventually food-gathering is shared equally between the parents, and is usually sufficient to produce a weight gain of 3 ½  ounces a day for male chicks, and 4 ½ ounces per day for the female chicks. (Female raptors are typically larger than the males.)  The chicks in these photos are approximately two weeks old and are covered with their darker, second coat of down, which comes in when they are a little over a week old.

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Bald Eagles Turning Eggs

3-16-16 bald eagles turning eggs 094Bald eagles are on their nests, incubating their one-to-three eggs. Although there isn’t much activity at this point, what activity there is, is extremely important to the survival of the embryos inside the eggs. Maintaining the temperature of the eggs is crucial. Bald eagles must incubate their eggs for 35 days. During this time, the eggs must remain at a steady temperature of 99.5 degrees Fahrenheit. If the incubating parent is frightened away from the nest or leaves the nest even very briefly, the egg may become too cold or too hot, depending on the weather.

During the entire incubation period, the eagle parents continuously turn the eggs with their feet and beak. The main function of this action is to prevent the embryos from sticking to the inside of the egg shell. In the early days of incubation, it’s important that the embryo floats inside the egg while the membranes that support its life are growing and developing. Turning optimizes membrane growth. Eventually the membranes will be pressed to each other and to the shell. If these membranes adhere too soon the chick will not be able to move into the hatching position later and get out of the egg.  To see a nesting bald eagle turning eggs, go to http://www.eagles.org/dceaglecam/.

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Carrion a Vital Food Source for Bald Eagles

3-1-16 eagle3 036Eagles obtain food mainly in three ways — by direct capture, scavenging for carrion and stealing food from other birds and mammals. When securing their own live prey, they hunt from perches or soar over suitable habitat, taking most prey on the wing. Bald eagles’ preferred food is live fish, but they are opportunistic foragers that select prey based on availability. Twenty studies from across their range found that the composition of bald eagle diets averaged the following: fish-56%; birds-28%; mammals-14%; and other 2%.

In addition to capturing live prey, eagles rely heavily on fish, bird and mammal carrion, especially during the winter. Ice fishermen’s leftover bait and/or rejected catches, roadkills and deer that have slipped and died on ice-covered ponds and lakes are three heavily-used sources of food at this time of year. If the carrion is small enough, it is often carried to a perch (see opossum in photo) where it is inconspicuously consumed. Larger carrion, such as white-tailed deer, salmon and waterfowl, that are too big to carry off, are eaten on site and repeatedly visited until consumed.

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Juvenile Bald Eagles Fledging

7-17-15 juvenile eagle 003For several weeks before leaving their nest, young bald eagles constantly flap their wings, occasionally lifting themselves off the nest several inches in mid-air. Eventually they succeed in making short “flights” to nearby branches and then back to their nest. The young eagles are strengthening their wing muscles, practicing landing, and beginning to master flight.

Sometime between 8 and 14 weeks juvenile bald eagles leave their nest. Many fledge successfully, but up to half of the nest departures are unsuccessful, with the young ending up on the ground where they may stay for weeks before flight is achieved. During this time the parents usually continue to feed their young, but the juvenile eagles are far more vulnerable to predators. If the flightless, grounded juveniles are approached or threatened, or if they simply want to move from one spot to another, they walk or run on the ground (see photo).

The young bald eagle pictured was blown out of its nest prematurely and landed on the ground. Eventually it managed to fly back up onto a branch in the nest tree. The parents continued to feed the eagle while it was on the ground, as well as after it was back up in the tree. (Thanks to Linda and Roger Whitcomb for photo op.)

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Young Eagles Preparing to Fledge

6-11-15 eaglet 062For several weeks prior to their first flight from their nest, Bald Eagle nestlings practice flapping their wings to the point of lifting themselves up several inches into the air. This develops their wing muscles, flight coordination and landing ability.

At anywhere from eight to fourteen weeks of age, juvenile Bald Eagles fledge, or leave their nest. According to Birds of North American Online, up to half of young Bald Eagle nest departures are unsuccessful. The young land on the ground and may remain there for weeks before regaining flight ability. More often than not their parents will continue to feed them, but they are much more vulnerable to predation in this situation. Fledglings may continue to use their nest as a feeding platform for several weeks after leaving it, as they gain flight and foraging skills.

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