Alaska Fish & Wildlife News
December 2004
Alaska’s Most Powerful Bird of Prey
By Riley Woodford
Golden eagles are found throughout Alaska, but breeding pairs prefer mountainous areas. photograph by Doug Backlund Golden eagles are formidable predators in Alaska. Bigger and more aggressive than bald eagles, golden eagles prey on young Dall sheep and caribou – and can take even larger prey. “ They ’ re built for killing, ” said Fairbanks wildlife biologist Jack Whitman, who studied gold eagles in Idaho. “ Comparing their forcible forte and food habits to bald eagles, the fortunate eagle is an aggressive, successful predator, and the bald eagle is a in truth good scavenger. How frequently do you see a fortunate eagle at a drivel deck ? ” Alaska Fish and Game biologist Steve Arthur found that golden eagles are significant predators of Dall sheep lamb in the Central Alaska Range. The lamb are born in recently May and they ’ re very vulnerable in their inaugural few weeks. “ The eagles – either soaring overhead or perched – constantly watch the bands of sheep, and if a lamb strays away from its mother there ’ s an eagle zoom in, ” Arthur said. “ They swoop toss off and try to grab it before it can get back under its mother. ” In defense, lamb hide beneath the ewe, and ewe will bunch up. In exorbitant terrain, they ’ ll catch under overhanging rocks and cliffs. golden eagles can fly 80 miles an hour and flush faster in a dive, but Arthur said they do best when they get a straight shot at their target – and sheep will use rough in terrain to hamper the eagles ’ approach. Golden eagles will besides team up to hunt, one eagle flushing out or chasing plot sol another can grab it. Arthur once watched a match of eagles work at separating a ewe from her lamb. “ One would harass her until she ’ five hundred charge off in that commission, and that would leave the lamb behind, then the other would come in at the lamb, she ’ d come back, always one working her, the other trying to grab her lamb. The inadequate lamb was fair spinning in circles. ” Arthur said eagles will take lambs into deep October, when they weigh more than 50 pounds, but it ’ s strange. Most eagle predation on lambs takes place early in the season. Rodents, birds and small crippled are by and bombastic the pillar for eagles, and in many parts of Alaska, that ’ s chiefly snowshoe hares. Hares are profoundly significant to eagles, and indirectly, to Dall sheep. Eagles and coyotes are the primary predators of Dall sheep lamb, but both predators are much more dependant on snowshoe hares. Hare populations fluctuate on about a ten-year hertz in the Alaska Range, and those cycles consequence eagles ’ generative success. Hares top out, crash over a biennial period, and then gradually increase over about eight years. Hare populations peaked in 1999, began declining in 2000 and hit bottom in 2001. “ We had low lamb survival when the hares were at their acme, ” Arthur said. “ My interpretation is that abundant hares lead to abundant predators, and when they ’ re abundant they ’ re not only eating hares, they ’ rhenium eating lots of early prey. ” Arthur said there is a meaning dangle in marauder generative success when hares crash, and basically they do not reproduce in years following the doss. “ Residents can persist, but they ‘re not able to reproduce successfully. They eat other types of prey that tide them over, but without that abundant source of protein can ’ t bring off a litter or a clasp. ” Golden eagles besides eat Arctic ground squirrels, marmots, gripe and ptarmigan. They will take larger prey, like beavers and foxes and are adequate to of taking large birds like fathead and cranes. In the Arctic, eagles are common predators of young caribou calves. Ken Whitten, a retire fish and Game research biologist, studied calf mortality in the Porcupine Caribou herd on the North Slope and Coastal Plain in the 1980s. Tens of thousands of calves are born in the jump, and Whitten and a veterinarian probe hundreds of calves to determine the versatile causes of end. He found that fortunate eagles rival bears and wolves as predators of caribou calves. early in the season, calf mortality was about 25 percentage. About half was from predation and half from congenital defects – the calves were born dead or died soon after give birth. But predators were not simply scavenging or even targeting the weak and dying. “ We found that predators were not taking the weakest calves, ” Whitten said. “ We looked at ( abruptly calves ’ ) adipose tissue reserves, and predators were selecting for goodly calves. We hypothesized that in that environment, predators are hunting by sight, and they select a moving prey rather than looking for one that ’ s already dead. There are thus many calves up and walking. even goodly calves are highly vulnerable to predators, they barely have to find one away from ma. ”
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Denali National Park is home to the greatest density of nesting golden eagles in North America. photograph by Doug Backlund The ways bears, wolves and eagles kill and eat caribou calves leave tell-tale clues. An eagle can carry a six- or eight-pound lamb, but it can ’ thymine carry off a 15- or 20-pound calf, and eats it where it lays without dismembering or skinning it the direction bears and wolves do. “ Eagles tend not to flush off a killing, even if you land a helicopter near them, or flee over with a airplane, ” Whitten said. “ But you see bloody-faced bears or wolves running away, or they may stand over it, or carry it off. They can eat a calf quick. Eagles corrode dull. They eat everything on one side, then flip it over, and it ’ s a lot easier to spot the carcase from the breeze. ” When eagles attack, they by and large swoop down with their feet extended and talons out. Three front talons face ahead and the back talon, called the big toe, comes in behind. They hit with one metrical foot around the back of the head or neck, and the other foot by the shoulder. They strike and then clench their talons. The big toe can penetrate the shoulder blade of a newborn calf, and Whitten has seen deep puncture wounds that pierce the lungs. “ The calves bolt for their moms and stand under their bellies when an eagle flies over, ” Whitten said. “ An eagle can make a toss off quickly, but it can ’ t defend the dead calf from the mother. sometimes we ’ five hundred see a cow stand over calf, with an eagle nearby on a tuft, waiting for the overawe to move aside. They ’ ra patient, and finally she ’ ll act on. ” Fairbanks wildlife biologist Rod Bortje has watched mother caribou defend calves against eagle attacks. “ If a mother saw an eagle coming she ’ vitamin d sample and defend her calf, ” he said. “ She ’ ll rear and strike at the eagle with her front feet, we saw that on respective occasions. She ’ ll stand over the calf, and when the eagle ’ south in a crouch it can ’ triiodothyronine react excessively well. ” Most eagle predation on caribou happens in the calf ’ south first workweek of life, Whitten said. “ If they can get past the beginning three or four days they can react pretty fast, they ’ re pretty alert. They can outrun a person in just a couple days. Eagles have killed calves into July, when calves weigh 30 pounds or more, but they are much more opportunist late when calves are bigger. ” Eagles may live 25 or 30 years, and like other top predators, experienced hunters develop techniques and strategies that may enable them to make ambitious attacks. Whitten said golden eagles have been documented taking adult pronghorn antelope, mule deer and caribou. But in the Arctic, Whitten found that adult and nest eagles tended to target grind squirrels, and the young birds were more active in calving areas. Adults are well at defending territories that provide abundant rodents, Whitten said, leaving the more challenge work of killing calves to younger birds. “ Adult eagles have learned to kill larger game, ” Whitten said. “ But I think it ’ s an strange erudite behavior that a few individuals pick up. ” Bortje once came across an adult caribou in early on April that had been killed by a fortunate eagle. Golden eagles migrate north in the jump, and he suspects this shuttlecock may have arrived on the North Slope a little early, before calving season, and was possibly athirst and a bite bolder than common. “ We saw the caribou with the eagle sitting on it, and we were able to back track and see how the whole scenario took plaza, ” he said. “ The eagle made multiple strikes. You could see the marks in the snow where the eagle had struck the caribou – it would fall and get up, and the eagle would land and then come at it again. We don ’ thymine know what condition of caribou was before, but I ’ ve seen these multiple strikes on yearlings, besides. ” Bortje said in some cases eagles will make recur attacks on an animal over the course of several days, wounding and sometimes blinding their prey before it succumbs. “ When they attack bigger animals, they like to go for the eyes, ” he said. “ For general hunt, a lot of time they grab the animal instantaneously, and the talons enter on both sides, between the rib, and puncture the lungs. then they sit and watch, and wait, or come second later. ”
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I am broadly interested in how human activities influence the ability of wildlife to persist in the modified environments that we create.
Specifically, my research investigates how the configuration and composition of landscapes influence the movement and population dynamics of forest birds. Both natural and human-derived fragmenting of habitat can influence where birds settle, how they access the resources they need to survive and reproduce, and these factors in turn affect population demographics. Most recently, I have been studying the ability of individuals to move through and utilize forested areas which have been modified through timber harvest as they seek out resources for the breeding and postfledging phases. As well I am working in collaboration with Parks Canada scientists to examine in the influence of high density moose populations on forest bird communities in Gros Morne National Park. Many of my projects are conducted in collaboration or consultation with representatives of industry and government agencies, seeking to improve the management and sustainability of natural resource extraction.