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Golden Eagles in Denali

What do Golden Eagles eat and what is their role in the food chain?

Golden eagles eat mostly small to medium mammals and birds. In western North America they eat a lot of rabbits, hares, and squirrels. In Denali, they eat mostly snowshoe hare, arctic ground squirrels, and willow ptarmigan (a bird similar to grouse. Sometimes they eat larger animals, like caribou calves and Dall's sheep lambs. Eagles may eat carrion (animals that were already dead), but not when they are raising their young.

The food habits of Denali golden eagles change as the seasons change. They eat mainly snowshoe hares and willow ptarmigan in March and April when they are repairing last years' nests in preparation for the nesting season. From May to August, nesting golden eagles eat many arctic ground squirrels, as well as snowshoe hare, ptarmigan, hoary marmots, and many other small to medium sized birds and mammals. During the winter, carrion probably becomes an important food resource.

Golden eagles are one of the predators, along with bears and wolves, at the top of the Denali food chain. This means that rarely do any other animals eat them. Golden eagle nestlings are vulnerable to predation, but predation rates are low probably because their parents build their nests on cliffs where mammalian predators, such as wolverine and marten, cannot reach the nestlings. Adult golden eagles are not hunted as food by any other animals.

How do researchers study golden eagle reproduction in Denali, and what is reproductive success anyway?

Researchers began a study of golden eagle reproduction in Denali in 1987 because there was very little known about golden eagle reproduction in Alaska. The Denali study is unique because it has been watching so many pairs of golden eagles (over 60 nesting areas) for so long (over 15 years). One of the first things the Denali researchers discovered is that Denali supported a very high density of nesting golden eagles. By studying golden eagles in Denali National Park and Preserve, Denali's scientists provide information for protecting the golden eagles of Alaska.

Denali scientists collect data to monitor the reproductive success of golden eagles at several times each year. In late April, scientists conduct surveys of golden eagles nests. They fly out in a small helicopter to the many nests in Denali that have been used over the years. They count how many of these nesting areas are currently occupied by territorial pairs of eagles and how many of these pairs are attempting to nest. Pairs that occupy nesting areas are often seen displaying, mating, building nests, and incubating eggs. These pairs defend their territory from other pairs, lone eagles, or potential predators, and are therefore called territorial pairs.

Denali scientists conduct this survey at a time when most pairs have completed their clutches (laid their eggs). During the survey they are looking for evidence that a nesting area is occupied and to see if the eagles have laid eggs. A nesting attempt is documented by observing an eagle incubating eggs on a nest. Denali's scientists are careful not to disturb eagles during the incubation stage, as this is a very sensitive time for the eggs.

In mid-July, about two and a half months later, Denali scientists return to the nesting areas where they had seen eagles incubating in April and they count the number of nestlings. By this time nestlings are at least 51 days old, and very few nestlings die after they reach this age. Counts of nestlings during this period provide good estimates of the actual number of fledglings produced in the study area.

Using these data, Denali scientists calculate statistics on the reproductive success of Denali eagles each year. All of the rates are reported as percentages (multiply the ratios by 100).

The occupancy rate - how many of the nests are being used by a territorial pair of eagles. This is calculated by dividing the number of occupied nesting areas (those with territorial pairs) by the number of nesting areas that were observed during the April survey.

The laying rate - how many pairs of eagles are breeding (as seen by nesting behavior) - by dividing the number of nesting pairs of eagles by the number of territorial pairs of eagles.

The success rate - how many of the nesting pairs have fledglings. The success rate is calculated by dividing the number of successful pairs of eagles (eagles that have fledglings) by the number of nesting pairs.

The mean brood size - is the average number of fledglings per successful pair calculated by dividing the total number of fledglings by the number of successful pairs.

Productivity - the rate of production of golden eagles (fledglings) by the population of golden eagles in Denali. Productivity is calculated by dividing the number of fledglings by the number of territorial pairs. The following table shows an example of the type of data collected by Denali's scientists each year.

The following table shows the calculated rates and productivity using data from the table above.

Denali scientists also count the number of prey animals seen each year as a way to measure the amount of food available to the adult eagles in the spring during the incubation season. In this way, they can determine if reproductive success changes as food supplies change.

Denali scientists found that in the years with plenty of food, eagles produce more nestlings than in years with little food. This makes sense as an adaptation for survival for a long-lived species. Producing eggs, incubating for 45 days them until they hatch, and feeding nestlings until they can fly and hunt on their own, requires a lot of energy from the parent eagles.

In years of low food supplies, it is better for their long-term survival that they lay fewer eggs, or that they don't lay eggs at all and try again next year instead. Other factors, such as wet and cold weather can also affect the reproductive success. For instance, adult eagles may not be able to hunt efficiently during rain and nestling eagles may require more food to stay warm during period of cooler, wetter weather.

Studying the Denali golden eagles and the factors that influence their reproductive success and survivorship is important because the National Park Service is dedicated to protecting the animals and the habitat within its parks and preserves. This is impossible to do if you don't know enough about them. This study helps the National Park Service understand golden eagle ecology and the factors that influence the survival of golden eagles. This study is the only long-term study of golden eagle ecology in northern North America, although Golden eagles have been studied further south for many years in southwestern Idaho. The Denali study helps other scientists understand factors influencing the survival of golden eagles in northern ecosystems.

Source:

Carol McIntyre, wildlife biologist, Denali National Park and Preserve

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