Hunting and dietary habits of the golden eagle
The golden eagle ("Aquila chrysaetos") is one of the most powerful predators in the avian world. One author described it as "the pre-eminent diurnal predator of medium-sized birds and mammals in open country throughout the Northern Hemisphere". Golden eagles usually hunt during daylight hours, but were recorded hunting from one hour before sunrise to one hour after sunset during the breeding season in southwestern Idaho. The hunting success rate of golden eagles was calculated in Idaho, showing that, out of 115 hunting attempts, 20% were successful in procuring prey. A fully-grown golden eagle requires about of food per day. In the life of most eagles, there are cycles of feast and famine, and eagles have been known to go without food for up to a week. Following these periods without food, they will then gorge on up to at one sitting. The powerful talons of the golden eagle ensure that few preys can escape them once contact is made. The talons of this species exert approximately of pressure, around 15 times more pressure than is exerted by the human hand, although some claim that the largest individual females may reach a pressure of . It has been claimed that the golden eagle can lift more than its own body weight in flight. However, other sources claim that a hare, marmot or deer calf weighing is a struggle for even a large female to carry and that prey much over would require favorably high wind conditions.
Hunting methods.
At least seven main hunting techniques are known to be utilized by the species, with many individual variations and the ability in most mature eagles to quickly (and sometimes cleverly) vary back and forth between methods depending on the circumstance. The first described is "high soar with glide attack", where the golden eagle soars at least above the earth. Once it spies a prey item, the eagle partially closes its wings and enters a long, low-angled glide which can carry it over distances of with the speed increasing as the wings close more. Just prior to impact, the wings are opened, the tail fanned and feet thrust forward to grab the prey, creating a booming sound, causing by the wings whipping against the wind, in the instant before the strike that sounds like a clap of thunder. This technique is used for solitary or widely dispersed prey found in openings, such as hares or ptarmigans. A variation of the high soar where a lofty perch is used instead of soaring flight has been observed.
The next major hunting methods is the "high soar with vertical stoop", which is used to attack birds in flight. Since they are outpaced and out-maneuvered by swift-flying birds, they can only usually attack slower-flying species such as geese and cranes. The golden eagle also requires a height advantage over their prey for this rarely observed technique to succeed. In one observed case, some Canada geese ("Branta canadensis") in Montana were able to avoid predation by a golden eagle hunting them in this way by collectively gaining flying height. In another observation, a golden eagle was successful in knocking a ptarmigan out of a flying covey in Scotland.
The next hunting method is the "contour flight with short glide attack", which is considered the most commonly utilized hunting method for golden eagles. This consists of a low-level quartering flight often at only above the ground so they do not break the sky-line when observed from the ground and they can hug the contours of the earth below. This method is useful for hunting colonial (often burrowing) prey such as ground squirrels, densely populated leporids or birds found in concentrations, such as breeding grouse or even seabirds. The individual prey item is apparently selected in a manner of seconds before the strike. If the first attempt fails, the eagle may fly around and attempt to ambush the prey again.
The next hunting method is the "glide attack with tail-chase", which commences with a low-angled stoop some distance from the quarry. The prey is then chased closely, whether a hare running evasively or a grouse in flight. The key to success is eagle's agility and the prey's inability to find cover. In one case, a flying greater sage-grouse ("Centrocercus urophasianus") was caught by a pair of eagles using this technique.
The next major hunting method is "low flight with slow descent attack". In this, the golden eagle quarters low below the earth and then gradually swoops down on the prey. This is used for slow-moving prey, such as tortoises and hedgehogs, or any prey item with a general absence of escape behavior. This includes any potentially dangerous prey items, such as rattlesnakes and foxes. When hunting mammalian carnivores, the eagle may hover for some time over the potential prey and not press the attack unless the mammal looks down.
The next is the "low flight with sustained grip attack", which is used for hunting ungulates. Here, the golden eagle flies over a herd of ungulates which in turn often huddle or break into a run. The eagle then selects it prey (typically young animals, though sometimes infirm or exceptionally healthy grown animals) and lands on prey's back or neck, talons gripping firmly, attempting to pierce vital organs or cause shock via a crushing grip to bone and cartilage. The hunting eagle typically rides its prey for several minutes with wings outstretched and flapping to maintain balance until the prey collapses, either as result of exhaustion, shock or internal injury.
The final major hunting method is the "walk and grab attack", in which the eagle walks on the ground and attempts to pull its prey out of cover. This has been used for pulling jackrabbits out of brush but has even been utilized to grab the young of large prey (i.e. deer, sheep and badgers) from under the mother's legs. Tandem hunting may be done regularly, especially with larger prey items. Reportedly, while hunting in pairs, the male golden eagle flies in front of the female at a higher elevation and usually initiates the attack. Breeding pairs have been recorded hunting jackrabbits cooperatively with one individual following the other at different elevations above the ground. The initial pursuer diverts the prey's attention by stooping while the second flies in unseen to make the kill. A study in Idaho showed that the success rate was lower during tandem hunting (9%) than during solo hunting (29%), but this may have been due to the more difficult nature of the prey targeted during tandem hunts. As a whole birds are reportedly more difficult prey to capture than mammals, so hunting success rates may be lower where birds outrank mammals in importance as prey. Golden eagles are not above scavenging for carrion. In fact, it makes up a significant portion (sometimes a majority) of the diet in winter, when ground squirrels are in hibernation and rabbits and hares tend to be at population lows. In the Greater Yellowstone area, the golden eagle was one of the most frequent scavengers to attend wolf kill-sites in winter but, unlike common ravens ("Corvus corax") and bald eagles, were not frequent at kills left out by human hunters. In many parts of the range (i.e. Alberta, Scotland, Spain, etc.) carrion was readily fed to the young during the breeding season.
Prey.
Golden eagles are opportunists and virtually any animal of a reasonable size may be predated. Well over 400 species of vertebrate have been recorded as prey. Prey selection is largely determined by the local availability and abundance of prey species. They normally prefer wild, native prey but can easily adapt to domesticated and/or exotic animals, most often when the prey returns to a feral state. This is most apparent on islands that have very few (or no) native land mammals, such as Corsica in France, Santa Rosa and Santa Cruz Islands in California and many of the Inner and Outer Hebrides of Scotland. Most analysis of dietary habits of raptors results from examining the prey remains around an active eyrie at the end of the breeding season (September or October), based on pellets, skeletons and skins found. This method is not comprehensive for prey analysis, as particularly small prey may leave no trace and it cannot take into account the prey selected by wintering adults and highly nomadic juvenile eagles (both of which are believed to include a higher level of carrion and large prey items). However, since eye-witness accounts of hunting and close monitoring of prey brought to the nest are both rare and likely to disturb the eagles, this is the best known method of studying the eagle's prey. Studies have revealed that the average golden eagle nest contains 3.57 species of prey, although there is considerable variation in the dietary breadth across the range, ranging from an average of 11.2 species in the French Pyrenees to a mere 1.4 in Alaska. In general, the dietary breadth is greater in Eurasia than it is in North America, where eagles frequently only need to hunt two or three species throughout the nesting cycle. This is a moderate dietary breadth by the standards of the genus "Aquila", and not remarkable when compared to some other raptors. Overall prey weight has ranged from to at least , although most prey taken are around half the weight of the preying eagle, with a typical prey weight range of but mostly in the lower half of that range. Studies have shown an estimated mean prey weight of for golden eagles across their range. Only 15.8% of prey weighs over . At a nest studied in Mongolia, prey items were found to be heavier than those known elsewhere in the range, with an estimated mean weight of around . The average estimated weight of prey taken by most other "Aquila" species is generally much lower but the wedge-tailed eagle seemingly takes prey of a similar weight (both in average and range of prey weights) while the Verreaux's eagle average prey weight is higher, probably over .
The most significant group of prey for golden eagles is mammals. In 59 studies of the breeding season diet across the range, 63.2% of prey remains were of mammals. The diet in North America is particularly skewed towards mammals with roughly 84% of prey items being mammalian. After mammals, other birds were most significant, comprising about 26.8% of prey. Reptiles comprised about 7% of prey from across the range, with other prey groups comprising the remaining 3%. All the diverse groups of prey are examined below, using mainly the aforementioned 59 dietary studies.
Leporids.
The family Leporidae (rabbits and hares) is easily the most significant prey group, comprising about 32% of prey known to be taken by golden eagles. Twelve out of sixteen dietary studies in North America showed leporids as the most common prey group, in some areas comprising over 75% of prey remains. In the northern Rocky Mountains and Northern Plains areas, the main prey species are the white-tailed jackrabbit ("Lepus townsendii") and the mountain cottontail ("Sylvilagus nuttallii"). In the Southwest, the Great Basin, and most of California, the main prey species are the black-tailed jackrabbit ("Lepus californicus") and the desert cottontail ("Sylvilagus audubonii"), the former being especially important, comprising about a quarter of North America prey items in a 1976 study. In two studies in southern Idaho, the black-tailed jackrabbit and the two cottontails dominated the diet, comprising over 70% of nest remains. More locally other jackrabbits and cottontails may be taken in smaller numbers. The snowshoe hare ("Lepus americanus") is thought to be the dominant prey species in the boreal forests of Canada but there have been no dietary studies conducted there. The snowshoe and the larger Arctic hare ("Lepus arcticus") are some of the most significant prey species in Alaska, though in one central Alaskan nest they were secondary to ptarmigan and nests in Denali National Park included only 0.4% leporid remains. Nests in Alberta and Washington had almost no leporid remains as well.
19 out of 45 studies in the Palearctic listed leporids as the most common family of prey for golden eagles. Important prey species include the mountain hare ("Lepus timidus") (in the Scottish Highlands and the French Alps), the European rabbit ("Oryctolagus cuniculus") (in the Scottish Highlands, Sicily, the Apennine Mountains, Switzerland, Spain, the