Bald Eagle Facts
Q&A with Peter Nye in 1999
New York Department of Environmental Conservation


Q. How many muscles do eagles have?

A. A very interesting question that sent me to the books! I found no specific reference to bald eagles, but the Audubon Encyclopedia of North American Birds says that 175 different muscles, most of which are paired, have been described in birds. The number in eagles is likely close to that. The legs alone contain about 35. Muscles make up somewhere between 35 to 60 percent of the total mass (weight) of birds.


Q. During migration, what is the average distance an eagle flies in a day?

A. Between 75-125 miles usually, but weather affects that estimate. Try to figure out how many miles some of our eagles went between fixes, then divide by the number of days between fixes to come up with an average daily movement.

Q. When eagles get ready to fly long distances, do they eat a lot for energy or eat less for weight loss?

A. Prior to migration eagles will eat normally; they do not "fast" for weight loss. They may even tend to eat more to load up if food is readily available. Many of our wintering eagles fly north to their nesting sites while much of those areas are still frozen. Since they largely are cueing their departure to daylight length, they do not know how harsh the winter might be up north (and hence how frozen everything might be), or for how long. Thus, they might well arrive back "home" without alot of food for several days or even a week. We have witnessed migrating eagles stocking up on food en-route also. If a migrating bird comes accross a deer carcass for example, it might hold up its migration for a day to feed up on the available food, kind of like eating now because you just don't know about tomorrow!

Q. Why are eagle nests so large for their body size?

A. Actually, eagles' nests are just about right for their body size. Most nests are about 6 feet across at the top, and with two adult eagles and one, two, or sometimes three young in the nest, it can get pretty crowded. Especially when you consider that as the nestlings approach fledging age, their wing span is six feet or more, taking up most of the nest. Nests can get very deep (one was recorded in Florida that was 22 feet deep!), because most pairs add sticks to the same nest each year, and use them for many years.

Q. Do bald eagles take the same migration route each year?

A. We are just now finding this out! Look at the path that New York eagles F43 and F44 took to their nesting sites in the spring of 1998, and compare that to their path this spring. What do you think?


Q. How big are the radio collars?

A. We do not use "collars", as they are sometimes used on bears or wolves. We use what are called "backpacks." The packs sit squarely on the back of the eagle between the wings. Straps run around the front of the wings and under the wings and are sewn together at the eagle's breast. When the cotton thread we use rots, the whole package falls off the eagle. Each pack weighs 95 grams. I could tell you exactly how many ounces it is, but it is a good exercise for you to look up how many grams per ounce and figure this out yourself! Once you figure it out, find another object the same weight and put it in your hand. How does it feel, heavy or light? Under national bird banding/marking guidelines that biologists follow, any radio transmitters may not exceed 3 percent of the bird's body weight. Our eagles weigh, on average, 11 pounds. Figure out what percentage of the average weight the transmitter represents. Is it more or less than 3 percent? If you carried 3 percent of your body weight, how much would that be?

Q. Does having a larger population every year create problems for eagles?

A. It certainly can. It is called "density", and depending on the density (how many eagles there are per given amount of suitable habitat) eagles may not be able to breed successfully or may not be able to breed when they are initially sexually mature at five years of age. When too many eagles are in a certain area, competition exists for limited resources (food and nesting areas), so not all the birds can breed. In such cases, you often find the clutch sizes (# of eggs and young) less than in sparse populations. In these areas fewer birds will have young, and each will have fewer young. Also, when too many birds exist for a given habitat or region, we can start to see them selecting nesting sites that are not the best quality, ones we call secondary or lower quality sites. Problems such as poor nest trees, excessive human disturbance, increased interactions/aggression between eagles, or other hazards can accompany such poorer quality sites. The best sites have already been taken by the strongest eagles. However, as with most wildlife, eagles would eventually regulate themselves, and would not simply continue to grow in numbers indefinitely.

Q. How can we know that new chemicals aren't hurting eagles like DD2 did?

A. People (like me and other biologists, ecologists or land managers) are now very aware of and on the lookout for contaminants, in water, on land, and in a variety of prey species all the way up the food chain. For example in New York, we routinely collect unhatched eggs from nests during eaglet banding, and have them analyzed for contaminants. We also routinely collect and analyze prey (fish) and often collect blood from eaglets and adults when we have the opportunity. Thus we closely monitor what is going on in the environment so we are not surprised as we were by DDT in the 1970's.

Q. How did the bald eagle survive the DDT problem?

A. Fortunately, we discovered the DDT problem in the nick of time. A retired banker/eagle enthusiast from Canada, Mr. Charles Broley was one of the first to notice a problem of non-producing eagles in Florida, followed by a famous scientist named Rachel Carson who wrote an alarming book about DDT and its effects called Silent Spring (you should all read this book one day !). These "alarms" were sounded in the late 1950's and early 1960's. Fortunately, DDT was banned in the United States in 1972, and eagles (and other raptors such as peregrine falcons and ospreys) began successfully reproducing sometime after that. Active restoration programs helped accelerate the eagle's return. Fortunately today, eagles can once again breed successfully and raise plenty of young to keep the US population growing. From the eagles nadir (low point) in the early 1970's and late 1960's when fewer than 500 breeding pairs were known within the continental US, by 1998 over 6000 pairs were breeding in this area. The important thing for us to do now, is to make sure we keep enough suitable habitat around for all these eagles to use now and still when you all have kids of your own !! Due to the significant increase in bald eagles over the past 20 years, the federal government will soon be proposing to remove the bald eagle from the threatened species list.

Q. Will an eagle remember you if you tease it or mess with it?

A. This is a difficult question, in that such behavioral responses by eagles or any species) to disturbances is tough to identify for sure many times, as to what exactly may have cause their behavior, or in the absence of our "seeing" any behavioral change, really knowing if our actions have had an impact on them or not. There could be some subtle reactions to our "messing" with them that we just aren't seeing. Whether our actions disturb them or not would depend on a couple of things; the type/severity of the "messing" (disturbance), and the frequency of the disturbance (how often it happens and how spread out or close together they are). All biologists are constantly concerned about this potential problem when we study wildlife. We do not want our actions to result in disturbance to the animals which might change their normal behavior patterns and thus bias or study of their habits. So far with this migration tracking work, we have not identified any such changes or negative impacts. That is one reason we try to really limit the size and weight of the transmitters.

Q. What is the furthest distance an eagle you've tracked has flown?

A. Approximately 1,100 miles here in the east, but check with our western US guy, Jim Watson. I think some western eagles have been tracked much farther.

Q.What have you learned since tracking eagles that most surprised you ?

A. Another excellent question! What has most surprised me is the path these birds are taking. Of about 20 or so that I have now tracked, they all seem to go in somewhat different directions and routes to get to their destinations. I remember when we used to try to track these birds from the ground using what we call "conventional" radio transmitters (rather than the satellite transmitters we use today). We would have to make a guess as to which way they went when we lost their radio signal, and then go like heck in that direction to try to pick them up again. Well, even in a small airplane, by this method, eagles gave us the slip almost every time! Now, looking at how each does something different (no real common patterns), I can see why; we didn't stand a chance! I have also been surprised at how fast they move out. Once they decide to go, they cover hundreds of miles in just days, again, making it impossible to keep up with them.

Q. Do eagles carry any diseases that can hurt humans?

A. No

Peter E. Nye
New York State Dept. Environmental Conservation
Division of Fish, Wildlife and Marine Resources
Delmar, NY
Spring, 1999