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Bald Eagle Facts
Q&A with Peter Nye in 2006
New York Department of Environmental Conservation

Characteristics

Q: How can you distinguish between adult male and female bald eagles?

A: Visually, they look identical, but as with most raptors, the female is larger (heavier and bigger) than the male. Sometimes this is clearly visible in a pair, when you see both together at the nest, but otherwise, we are just guessing. In the hand, biologists can differentiate male versus female using two key body size measurements, the depth of the bill (beak) and the length of the hallux talon, on the rear toe. These measurements are plugged in to a neat formula developed by eagle biologist Gary Bortolotti back in the early 1980's, based on numerous measurements of eagles of known sex.

Q: Do eagles see in color or black and white?

A: Great question; congrats to you! Everyone knows bald eagles have superior eyesight. 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 vision, versus the "rods" which are for sight in low-light conditions, something eagles are not especially adapted to.

Q: Why are the adult eagles' heads white?

A: That's a great and logical question! While no one knows the answer for sure (not scientifically proven) below are some hypotheses:

  • for visibility, to makes it easy to locate and identify other individuals of the species
  • for denoting sexual maturity, versus immature eagles, which lack the white head
  • for indicating dominance, like top-dog

Life Cycle

Q: Will eagles choose a new tree if their nesting tree falls down?
We had a pair of eagles with a nest in a large tree near the Missouri river near Nebraska City. This winter the tree went down. We're worried the pair won't nest here again. We did see an eagle on the ponds nearby, on the ice eating something.

A: Sorry to hear the nest tree blew down, not an uncommon occurrence! Not to worry. Eagles are very faithful to their nesting territory, not necessarily to the actual tree. I don't know how long the eagles have been nesting there, but I would fully expect them to build a new nest not too far away. It could be up to a mile, rarely further, but I'd suspect even closer, all other conditions (like food) being equal. Watch for them carrying sticks off in a certain direction.


Ecology

Q: What's the average distance an eagle flies per day? Does it vary between male and female?

A: Well, as with other questions, this varies; there is no one answer to this. This is very dependent on the time of year the availability of prey, weather conditions, age of the eagle, and many more things. Obviously, during migration, for those eagles that do migrate long distances each spring and fall, average daily flights can be 200 km or more (124 miles). While on the wintering and breeding grounds, flights are much shorter, and depend heavily on prey availability, suitable roosting locations, and weather. I would say on average, daily movements during non-migratory times would be on the order of less than 10-20 miles per day, if that. Regarding differences between males and females, I don't think there would be many in winter, but in breeding situations, the males will travel farther and do more of the food provisioning, while the females will stay closer to the nest, even when they are relieved by the male at the nest.

Q: How much does an eagle eat in a year?

A: Another great question from Ferrisburgh! To answer this one, we have to look at how much energy (food) an eagle requires on a daily or weekly basis, and then we extrapolate for an entire year. Some factors affect the raw amount of food an eagle may require: some foods are of higher "energy" value than others; what activity the eagle is involved with (flying, perching, egg-laying, etc.); and time of year - eagles may require more food at certain times of the year, such as in winter when they are losing more energy. And, of course, the size (weight) of the eagle plays a role; smaller males will not need as much food as a larger female. Also, smaller eagles, like those from Florida versus their much larger northern cousins, will require less food due to their lower body weight. As with all creatures, finding enough food (energy) is the key to survival. It has been reported that eagles energy needs range from about 450 - 550 calories per day (about 15-25 percent of what humans require per day. Based on average human weight versus average eagle weight this must mean eagles are a heck of a lot more active than the average human!).
Various studies of eagle-energetics suggest that normal food intake for an eagle is somewhere between 6-10% of its body weight. So, assuming the average weight of an eagle to be 10 lbs., it would translate to between .6-1.0 lbs of food per day, or between 219-365 pounds of food for a year.

Q: What survival instincts help the Eagle most?
For example, what social/group dynamics does it rely on? What communication? What hunting process?

A: You have to look at the overall ecology of bald eagles to adequately answer this one, and tear apart the species various life functions. Actually, this is a great question to turn back to kids (this one sounds like it came from an adult). Based on what you've learned about bald eagles so far (and hopefully you kids have already looked up some general information on this magnificent species), what characteristics do you think help the eagle survive?

One could just about write a book on this one question. To keep it short, I'll give you just a few of my thoughts; add some of your own, and even pick another species of your choice and make a similar list of what you think are its strategies for survival. Through many generations of evolution, each species is uniquely suited for survival, in its own special way. This includes what some might consider the lowliest invertebrates. This is natures way. We call it unique adaptation. How species cope with their surroundings is what survival and procreation is all about, including for humans! Anyway, on to the eagles: The question first asks about "instincts", which maybe wasn't really what the asker meant, but we'll start there. Look up "instincts". Instinctively, bald eagles do many things that must be adaptive (a positive trait) for their survival. I'll provide some traits and you figure out why these might aid their survival:

  • they typically build their nests high up in trees, almost always with commanding views of their surroundings (all the better to avoid and see my enemies)
  • they typically build their nests near water (all the easier to spot and access my food)
  • they vigorously defend their nesting territory when breeding, yet become gregarious (look it up) in winter
  • they are sensitive to and avoid disturbance
  • they will build and use alternate nests
  • they will sit (perch) for long hours; in winter, for up to 98% of the day (so I can conserve precious energy)!
  • they will select winter night roosts that meet very specific criteria (slope, aspect, etc.)
  • they will eat carrion!

These are just a few of the "instinctive" actions eagles use to survive; there are many more. Here is a partial list of "traits" that bald eagles possess, that aid in their survival, but are not "instincts":

  • their eyesight
  • their specially adapted legs, feet and talons
  • their distinct plumage, especially that of young birds (mostly brown and cryptic)
  • their size
  • their delayed sexual maturity and long-life span
  • the larger size of fledglings
  • These may give you food for thought about how they enhance an eagles survival.

Q: What is the most abnormal behavior you ever saw an eagle do?

A: This question made me stop and think. It is hard to describe any behavior as abnormal, who's to say? It would imply we know everything about eagle behavior.

I did come up with a couple of our most interesting observations: In one case, during the breeding season, we watched a pair of bald eagles who had previously bred successfully at this particular spot. They began to systematically and methodically "de-construct" their perfectly good nest. Stick by stick, they removed the nest until nothing remained, flying off somewhere with the sticks! This may have been caused by competition with another avian trying to use their nest (great horned owl?), or because of competition at the site from ravens, which were nesting on the cliff below and were very defensive, or ?? Who knows for sure? What I do know, is that we see this pair still, regularly, but have yet to find their new nesting site!

A second incident is one that brings a smile to my face every time I tell it. At one of our oldest nesting sites in western NY, a raccoon accessed the nest after hatching and killed an advanced age eaglet in the nest; not an uncommon occurrence and the reason we place predator guards on all of our nest trees. As the eaglets get older, the adults don't stay on the nest at all times. The very next year, one of our observers watching the nest, actually spotted a raccoon again climb the nest tree and get to the nest containing a several week-old eaglet. Again, the adults were not on the nest at this time, but one was perched down the hillside not far away, and saw what was going on. Our observer watched as the adult eagle flew to the nest, snatched the raccoon off the edge, flew off a way with it in its talons, then dropped it from the sky! Revenge of the eagles! Knowing how hardy raccoons are, I imagine the tough critter simply bounced on the ground, shook it off and walked away.

Q: What's the normal route for bald eagles to Manitoba, Canada?
I live in Minitonas, Manitoba, Canada and for the last two years we have seen a bald eagle in our trees, always around September. This year, the bird was spotted in September and November, for about a week at a time. Last year (2004) the eagle killed a duck in our pasture pond, should we be worried?

A: I actually found Minitonas! Manitoba looks like a beautiful Province, and one full of lakes, rivers, and remote places that would seem perfect for bald eagles. I have to admit, I don't know a lot about bald eagles in Manitoba, but I have to believe you have a large breeding population up there, especially in the northern part of the Province. I do know that your surrounding Provinces, Ontario and Saskatchewan, have sizeable eagle populations, as does the Northwest Territories north of you. From your location on the map, and the presence of such huge, undeveloped lakes immediately to your north (Lake Winnipegosis, Cedar Lake), and from the timing you describe, my guess would be this is one of your local birds simply beginning it's southward movement away from the breeding territory and finds this a good place to stop. It may well continue much farther south in later November or December, to more suitable over-wintering areas in the US.

While eagles will take a variety of prey, especially if easily available and especially during fall, winter, and spring, you needn't be worried about anything; they usually prefer fish and will not kill all your ducks!

Q: Where do the Bald Eagles come from that reach Shageluk, Alaska in March? We live in Shageluck on the Innoko River. We did some research, thinking our Eagles came from Southeast Alaska but we were told that extensive research had been done on the Haines, Sitka Eagles and that they are not a migrating bird that flies very far from Southeast Alaska. Our response from the Raptor Center was that they might possibly come from the Aleutians. What is your opinion?

A: I found Shageluk on my map, and based on the timing you mention in March, I have to believe you are seeing eagles moving north to breeding grounds from wintering areas to your south. I have to admit I am certainly not an expert on Alaskan eagles or their movements! My guess would be that your birds might well be coming from the large winter gathering around the Cook Inlet/Kenai Peninsula area southeast of you. US Fish and Wildlife Service biologists out of the Juneau office know more than anyone about Alaska's eagles, so I would suggest contacting them.


Other

Q: How long does it take you to get to the nests and band the baby eagles?

A: This depends on where the nest is located (how remote), what kind of a nest tree it is, and how the nest is situated in the tree.

Sometimes, with very remote nests, it can take us all day of driving, boating and hiking to get to just one nest. Others may be close together and easily accessible from a road so we can do up to 4 and sometimes 5 in a single day. Some trees are quicker and easier to climb than others; pines are pretty nice, although sticky!
Some very large deciduous trees (look that word up!) are very high and difficult to get up, especially when the nest may be located way out on a nearly horizontal limb, versus right on or next to the main stem. Other trees, I hasten to add, are LOADED with poison ivy vines, with woody vines up to 3-4" think winding up the tree for more than 60 feet. In those cases I have to slowly work my way up, cutting vines as I go, which takes a LONG time! In some other cases, we shoot a small rope up over the first limb, then pull our climbing rope up, then mechanically "ascend" the rope, rather than climbing the tree. Obviously this takes quite a bit of time to do

Q: What about the Eagle do you think makes it an appropriate symbol for our nation? What are the most impressive virtues of the Eagle? What are its limitations and how does it compensate?

A: This is a very individual question, one each person almost has to answer for themselves. To me, bald eagles are the perfect choice for our nations symbol:

  • they are unique to North America, making them "our" eagle
  • they are strong and independent
  • they are survivors (they'll eat carrion if they have to, they'll steal fish from ospreys, they'll kill intruders and defend their territory), just like Americans!
  • and, of course, they are majestic looking!
I think one of the biggest limitations eagles have, especially adult bald eagles, is their visibility (their size and coloration). They overcome this in a variety of ways (see above), by, for example, being very sensitive/wary to intruders/disturbance, building their nests with vertical isolation, and of course their super-keen eyesight.

Q: Could we ever re-populate Bald Eagles on the Charles River-Boston or Jamaica Bay-New York City? I'm the volunteer chair of the Urban Ecology Institute at Boston College. Please check out our efforts and explore how we could work together in the future. Our focus is to look at urban eco-systems and renew them through low-income, inner city high school student field biology research experiences and research translating into policy recommendations.

A: Your repopulation idea in those two areas would depend on a variety of environmental and sociological factors. Neither area to me sounds suitable for bald eagles.

Q: Why do you think eagle P91 traveled so far south away from the others?

A: P91 is a male eagle that fledged from a nest along the Hudson River just north of Albany (Eastern NYS) in July 2005. His movements are interesting to study. He departed his natal area for the first time around the 21st of August, about 6 weeks post-fledging. His initial trip took him north, up into the Adirondacks of New York. From there he continued north into Canada (Ontario) in mid September. By early October he had done a reversal and gone south into Pennsylvania, and by early November, farther south yet into West Virginia! This area seemed to be to his liking through the winter until early March when (thankfully, as I said in some earlier notes) he moved significantly east into the Chesapeake Bay area. Very recently, as you'll see in the 15 March posting of data, he has moved again, not far, but just to the east a bit into the northern portion of Delaware Bay. Why? This is really hard to say, especially for juvenile birds, but his patterns are actually fairly representative of many young eagles; moving north, then with onset of winter, moving south. As you see from our data of the nestling movements, several have traveled quite a bit south, into the Chesapeake area, so P91 is not that different. He was a bit more inland than any of the other birds, and stayed pretty much in the same area for quite a while through the winter, hence my relief when he finally moved (I was concerned something happened to him). As I've said before, young eagles (especially the young males), travel wherever they want. We characterize this behavior as "random wandering" compared to adult birds, who have established routines and regular places they visit in their travels each year. This is all undoubtedly part of the learning process for them, as they prepare for sexual maturity and the rest of their lives.

Q: Is the eagle on the additions page (the one that has a blue band on the left leg) one of your eagles? There is a photo gallery of a recent attack by an adult Bald Eagle on an Egyptian goose (probably escapee) on the Hudson River at Coxsackie. On the additions page the third eagle picture shows a silver band on the right leg and the fifth eagle picture shows a blue band on the left leg. Is there any way to tell without the band numbers? When I arrived at the Coxsackie boat launch this morning, I soon saw a flying adult Bald Eagle in pursuit of another.

A:
Yes, this eagle can be confirmed to be a New York State eagle, based on the clearly evident blue band on the left leg. All (and only) New York State eagles get these sky-blue bands. Without being able to read the actual alpha-numeric on this band, however, we cannot say what individual it is or where it came from. You'd think such a great photographer would have been able to capture the band number!

Q: How long does it take to catch an eagle? Is it easy or difficult? Do you have good luck every time?

A: Again, this question depends on where you are trying to trap, what you are trying to trap (one or both of a breeding pair versus just one of many in a wintering population), or if you are after a specific eagle. I imagine you are asking about our winter trapping efforts to try to get adult birds for our tracking studies, so I'll answer that. I'd say on average, it takes us about 6-8 days to get our "target" bird. Of course, if winter is harsh, and eagles are food-stressed, they are more likely to come to our bait. No, we certainly do not have luck every time! That would make this too easy! Some of the most difficult trapping, is when we are trying to catch adults on their breeding territory in summer.

Q: Do the golden eagle babies look different from the bald eagle babies?

A: Yet another great question from Ferrisburg!
Yes, golden eaglets look different than bald eaglets. When first hatched and as young nestlings (before feather growth starting at about 4 wks of age), golden eaglets are mostly white. Bald eaglets are much darker gray. Also, golden eaglets have a very noticeable yellow "cere" at the base of their bills, all through their nestling stage. Bald eagles do not; balds are uniformly dark. As golden eaglets age, they maintain a much lighter, whiter head than bald eagle nestlings. As they age they attain their very distinctive "golden" nape from which their name derives and which is the most obvious difference in older age eaglets. Check out some photos of both on the net and see for yourself!


Peter E. Nye
New York State Dept. Environmental Conservation
Division of Fish, Wildlife and Marine Resources
Albany, NY
Spring, 2006

 

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