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Bald Eagle Facts
Q&A with Peter Nye in 2001
New York Department of Environmental Conservation


Characteristics

Q. How much time passes between when the bald eagle sees its prey and when it catches it?

A. This would depend on how far away they spot the prey and when they decide to go after it. There is no automatic rule of thumb. Eagles will often sit for hours scanning the water for prey to either swim near the surface or float by. Generally though, once they leave their perch for some target prey, they either catch it or they don't within a minute or two. This also depends on the type of prey they are after. For example, if they are hunting waterfowl like ducks or geese, they may make many passes over an hour or two before selecting their target and killing it.

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!


Ecology

Q. What determines when adults migrate or stay put? We live in downeast Maine and have an eagle nest on our land. This land has a conservation easement on it so the nest is protected! The nest is near a salt water bay. We know that this eagle pair stays here through the winter.

A. Excellent! You are SO lucky! Maine has been doing an excellent job monitoring and managing/conserving their eagles, and many states could take a lesson from what has been done there. Anyway, to your question. You are again correct in your observation about "your" local pair. We pretty much know that eagles that breed on or near areas of water that stay open year-round, like the ocean or fast-flowing rivers, remain at or near their breeding territories all year. The key to whether they move or not? FOOD ! If then can get all they need to survive through the winter, why leave?

Q. If an adult pair migrate, do they stay together through the migration? If not, how do they reestablish the pair bond when they return to the nest?

A. Our research to date indicates that mated pairs of eagles that do migrate, do not migrate together. I'm not sure why this is, but it could be adaptive (beneficial to the species over the long run) for them not to migrate together. This reduces the chances of both birds being killed for some reason along their migration, and may ensure that at least one of the adults survives and returns to the breeding territory to keep it going. Although we do observe what seem to be "pairs" of eagles together on the wintering ground, we do not know if this is generally the rule, or if mated pairs may not only not migrate together, but may also not use the same wintering areas. Again, this could be adaptive. This is also one of the ongoing research questions we are attempting to answer, and given enough time, we will.

After the eagles return to their nest, separately, they re-establish their pair bond by time-spent together and through distinct behaviors, such as beak-rubbing and aerial courtship flights, which are spectacular, and consist of flying together extremely closely, chasing and tumbling like fighter planes.

Q. If the wings are hurt and it can't migrate, how long would an eagle be able to survive?

A. It would depend on how badly injured the eagle's wings were. I suspect that if an eagle had a wing injury so severe it couldn't migrate, this would mean it couldn't fly well and it would not survive for long, perhaps a week or two at the most if it could not fly and get food on its own.

Q. Eagles have been seen in New Jersey. How many pairs are now living there and how far south have they migrated?

A. For the most accurate answer, I would contact the NJ Wildlife, Endangered Species people in Trenton. But here's my guess; about 20-30 nesting pairs. I also don't believe they are very migratory. I bet they stay in NJ around their nest site or not too far away most of the year; some (especially the immature birds) undoubtedly move down to the nearby Chesapeake Bay.


Other

Q. What chemicals other than DDT are now affecting bald eagles?

A. Several chemicals/compounds are a concern throughout the United States. Two metals, lead and mercury, as well as compounds known as PCB's (polychlorinated biphenyls), are in our environments and we regularly screen for all of these. There are 209 "types" of PCB's called congeners, which we also look for in prey, blood and eggs. Lead is often picked up while eagles consume other prey with lead shot in them, such as waterfowl, geese, or sometimes in deer carcass'. PCB's and mercury are picked up through the food chain, mostly through/in fish. We are still concerned about, and still find DDT (and its breakdown products DDD and DDE) in eagle samples, even though it was banned nationally in 1972.

Lead can debilitate and kill eagles. Mercury can cause eggs not to hatch. PCB's can cause birth defects, and may reduce hatchability of eggs and/or survival of nestlings. Very high amounts could kill eagles directly.

Q. Where are the best viewing sites for bald eagles?

A. There are numerous places to view bald eagles around the country! Alaska would have to top my list, Southeast Alaska in particular, but also a place called the Kenai penninsula. I think by far, the most premier place to watch eagles in the fall and early winter, is the Chilkat Valley in Haines, Alaska, where thousands of eagles concentrate each fall to feed on stranded fish.

The Klamath basin in Oregon is a spectacular place to observe wintering eagles leaving their roost and flying to their feeding areas.

In New York, there are several viewing sites set up specifically to watch bald eagles, both winter and summer, including a public viewing blind at one site where hundreds of eagles gather each winter in southern NY, in an area where some of our eagles are captured for tracking each winter.
Check out your local Fish and Wildlife Agency.

Q. I am studying a group of Harbor seals that haul out on rocks very close to the eagle nest which means I can report observations of the eagles as well. Do you have any use for these observations?

A. You don't say where you are from. East coast I assume; Maine? I cannot use your observations, but perhaps your state fish and wildlife agency may be interested in your observations at the eagle nest, especially things like when they go down on eggs, hatch young, and if the young survive and successfully fledge from the nest. I am sure they would also be very interested in any disturbances or problems you might witness at the nest site.

From a more scientific standpoint, I would be interested to know if you have ever seen the eagles try to harass the seals or get prey from them. How do they get along ? Any interactions between these species?


Peter E. Nye
New York State Dept. Environmental Conservation
Division of Fish, Wildlife and Marine Resources
Delmar, NY
Spring, 2001

 

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