Friday, November 13, 2015

How Much Can A Bald Eagle Carry?

On November 4th, photographer Alex Lamine photographed the female bald eagle at Berry College carrying a very large stick. The Berry College Eagles Facebook page posted: "Today, photographer Alex Lamine caught an extraordinary occurrence at the nest. Around 7 a.m., as morning light began to creep into the sky, Alex watched Mom Berry gnaw a limb off a tree and begin flying it toward the nest tree. She carried the long limb in her good (right talon) and Alex captured the action. Immediately after, Mom dropped the limb and it landed just a few feet from Alex. It impaled the earth on the heavy end and could have caused a serious injury. Eddie Elsberry weighed the limb later today and it weighs 12 pounds! We've seen both eagles drop limbs from time to time but this is the largest by far that we have seen. Thanks, Alex, for sharing your experience with us. It is truly amazing that she could carry such a heavy limb in one talon!"

 We immediately started getting questions about the stick. Could an eagle carry twelve pounds? Was she carrying the stick, or was it actually falling as she held on to it? We've speculated quite a bit about how much weight eagles could carry, but I decided to skip the musing this time and ask some experts. I ended up talking with Professor Jim Grier (Grier has studied birds of prey extensively and owned a golden eagle), Brett Mandernack, Neil Rettig, Chuck Sindelar (a good friend of Bob's who was deeply involved in bald eagle recovery), Jon Gerrard (who wrote The Bald Eagle: Haunts and Habits of a Wilderness Monarch with Gary Bortolotti), and Professor David Bird (among other things, the author of The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds). It was quite a group of experts, to be sure!

Several of them noted that the amount of weight a bird could carry was highly dependent on the situation, including:
  • Wind direction and speed
  • How the object was carried. Was it streamlined or catching the wind?
  • How dynamic the object was. Did it dangle and swing, or stay steady?
  • How the object was captured. Was it caught and carried in flight (with momentum) or dead-lifted from the ground?
  • The direction the object was being lifted in. Was the object being lifted up or down?
Professor Grier compared eagles with aircraft and talked about dynamics, flight conditions, and how the object was carried: "I sometimes got the impression that my golden eagle could carry as much or more in his crop than in his feet, but never got any good measurements or wrote down details. For example, he could capture a large rabbit that he could not fly with...but then he'd eat most of it and be able to fly with a really full crop. I think the trim of the eagle, as in airplanes, and drag/balance of carried items, is important." He added: "Flight conditions make a big difference, particularly at the limit of weight, as with aircraft. The best conditions are high air pressure with a steady wind plus room and conditions for a good take-off, all of which affect the ability to get airborne and then stay airborne. I've seen bald eagles carry large fish under some conditions, for example, that they couldn't under other conditions."

Think of an airliner. Cargo is balanced around the center of gravity inside the plane, not dangling below it. According to the FAA, center of gravity deviations as small as three inches can dramatically change the handling characteristics of some fully loaded aircraft. When Jim's golden eagle ate a large rabbit, it was for all practical purposes 'balancing' the cargo in its crop, which is located in the center of its neck above the top of its chest. In this case, balance was a bigger issue than weight as far as lift and stable flight were concerned. For more information on how the center of gravity affects flight, watch this video: https://youtu.be/00CGAPOp7A0

Chuck Sindelar wrote about watching golden eagles play with sticks outside of nest-building season. "They would fly off with a stick and gain altitude by circling. A second golden eagle would follow the first up into the sky and would get into a position where,  when the first bird with the stick dropped it, the second bird would dive and regrab it, often before the stick reached the tops of the trees below. Sometimes he would have to pull out of his dive and allow the stick to hit and enter the trees, and then both birds world move a bit away and do this all over again with a new stick." He also pointed out that while he had never personally seen a bald or golden eagle carrying sticks that large, he had often seen them in nests.

Jon Gerrard expressed interest in the stick gnawing and how the eagle ended up with the small end. In his observations: "...it would be more common for an eagle to fly in to a limb, usually a dead limb, and break it off in flight.  Did the eagle gnaw it off at the thickest part of the limb?   If so, how did it end up holding the thin part of the limb in its talon?   Did it gnaw it part way through at the thick end, and then fly to grab the thin end and fly with it to break it off at the thick end?  And how far did it carry it before dropping the limb?" He recounted a story from the book "The Bald Eagle Haunts and Habits of a Wilderness Monarch", which he wrote with Gary Bortolotti (pages 35 and 36)

"It is of a pair of eagles nesting along the Gulf Coast of Louisiana in the 1890s.  The female regularly caught and carried Snow Geese (Blue phase) - probably weight 4.5 to 6 pounds - for up a a mile and a half to their nest.   The Geese were caught high and the eagle was able to glide down to the nest with the goose.   This feat was repeated on a number of days, and there were apparently 35 blue goose heads found in the nest, suggesting it was a common practice.    The feat was likely possible because the geese were caught high in the air and going to the nest was downhill.   Since this eagle was nesting in the southern part of the range, the female's weight would likely be in the 8 to 11 pound range, so the bird would likely have been carrying about half its weight."

 Neil Rettig shared some of the observations he made filming bald eagles along the Mississippi river: "Last week I was able to get a nice shot on video of a bald Eagle near Stoddard collecting a big branch from the canopy of a cottonwood tree.  I have seen this many times.  I also filmed a juvenile bald eagle 2 winters ago catching an adult mallard in the air and having a hard time keeping it aloft. In high winds eagles can lift more, as Jim pointed out.  In winds they can hover for long periods 30 seconds or more to attack and work ducks and coots."

 In general, the expert panel felt that in most circumstances, it would be unlikely for a bald eagle to carry much more than 50-60% of its body weight. However, it might carry more if the incentives and flying conditions were right: favorable winds, a down-carry versus an up-carry, a momentum capture versus a dead-lift, plenty of maneuvering and flapping space, a well-balanced load, and a highly desirable object like a large stick or a dead fawn. Jon's questions about how the Berry College female ended up with the end of the stick are very interesting when considering load balance. Did BCF end up with the thin end of the stick accidentally or on purpose? It isn't the end she gnawed.

Professor Bird provided a table from his book, The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds, which I found absolutely astonishing! According to his information, a bald eagle should be able to carry 108% of its body weight. Check out the American kestrel at 145%, the Pallas's Fish Eagle at 160%, or the tiny Calliope Hummingbird carrying its mate - 116% of its body weight!

                                        SUGGESTED WEIGHT-CARRYING CAPACITIES OF BIRDS
approx. body weight (g)*
item carried
approx. weight of item (g)
percent of body weight
House finch
21
cloth rag
5
23
American kestrel
165
rat
240
145
Chestnut-collared longspur
20
nestling
14
70
Calliope hummingbird
2.5
mate
2.9
116
Osprey
1,800
fish
1,800
100
Pallas’s fish-eagle
3,700
carp
5,900
160
Bald eagle
6,300
mule deer
6,800
108
Golden eagle   
4,309
UID prey item
900
21
Harpy eagle
9,000
sloth
5,900
65
Steller’s sea-eagle
8,600
seal
9,100
105






Table used permission of Professor David Bird. Taken from his book, The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds

 * In all cases, a maximum weight was assigned based on the literature
SOURCES: B.P. Martin,  World Birds. (Enfield, Middlesex:  Guinness Books, 1987); J.  Terres,  ed.. The Audubon Society Encyclopedia of North American birds (New York:  Alfred A. Knopf, 1987).

Does this mean that a bald eagle can always carry 108% of its body weight?  No. But if the conditions and incentives are right, yes! The Georgia State Parks website states that bald eagles weigh 8 to 12 pounds and as watchers know, females are larger than males. So based on Bird's table, a 12-pound stick should be within her carrying capacity if the conditions are favorable.

As watchers also know, BCF dropped the stick. I would guess - and this is speculation! - that she had ideal conditions for take-off, including favorable winds, plenty of room, and a good spot for a drop. However, her load was poorly balanced. When she took off, she was carrying the stick with the heavy end up and to one side. It would have swung rapidly to hang below her, causing her to roll and pitch, and decreasing her lift. Sudden center of gravity changes are never a good thing, especially when the object you are flying with could weigh more than you do! She may have dropped the stick in response, or it could have torn itself from her grasp given its momentum. Either way, it was something to see!

Thanks so much to Jim Grier , Chuck Sindelar, Jon Gerrard, David Bird, Brett Mandernack, and Neil Rettig for talking to me! I feel very honored to learn from such accomplished and intelligent people. Any mistakes in the information presented here are mine. Brett also had some interesting thoughts on eagle vision and night flight that I am saving for another blog.

A few links - some for learning and some for fun!
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