Birder’s Notebook: Golden Eagle - An early bird
As I type these words in late April, the exciting rush of migratory birds back to the region is right around the corner. However, one species–the golden eagle–has been here for weeks, soaring over our icy landscape, proclaiming its presence to other arriving eagles.
By mid-March, golden eagles reoccupy their breeding territories scattered across the Seward Peninsula near the inland and coastal cliffs and tors where they nest.
It is a great time to appreciate the eagle’s aerial prowess as they engage in spellbinding courtship and territorial displays with dramatic steep, wing-tucked dives and rapid ascents, or dazzling aerobatic maneuvers, including back-flips and rolls, and occasional mid-air talon-locking flight with their mate. These big birds are aerial superstars.
Golden eagles form long-term pair bonds, but migratory populations such as ours are thought to go their separate ways in winter and reunite on the breeding grounds each spring.
Early arrival gives the pair time to court, reestablish pair bonds and build or repair their bulky stick nests before laying eggs in April. A pair usually builds and maintains a number of alternate nests within their territory–insurance in case a favored nest is still snow-filled, destroyed or already occupied by a gyrfalcon when they arrive.
Few species are likely to compete with eagles for their nests, but retired Nome raptor biologist Peter Bente says it is the gyrfalcon that can successfully defend its occupancy of a usurped eagle nest, making the eagles move to a new location.
According to Alaska Department of Fish and Game raptor biologist Travis Booms, ADF&G monitors raptors within an approximate 60mile radius of Nome. Surveys have been flown in this area since the 1970’s and nests have been monitored annually since about 2000.
Booms reports finding as many as 40 occupied golden eagle nesting sites in the survey area in a single year, but both he and Bente emphasize that site occupancy and nesting success vary wildly from year to year depending on weather conditions and prey availability.
Bente recalled a couple “breeding-bust” years when he found only about 10 percent of the average number of pairs occupying nests. Booms reported that last year was one such year when golden eagle nest occupancy and productivity were very low. He cited low availability of snowshoe hares in most areas and unseasonably cold, wet weather in the first half of June as likely reasons for nesting failures. Booms emphasized that annual variation is normal and no reason for alarm.
Golden eagles build their nests with sticks which they break from trees and shrubs, using finer vegetation and grasses as a lining. New sticks are usually added each year, so some nests become impressively large over the years.
Most often the female lays two eggs. She usually does most, or all, of the incubating and brooding. Her mate brings her food which she feeds to young chicks and eats herself.
The eggs are laid three to four days apart and incubation begins when the first egg is laid. Thus, one egg hatches first, so that the older, larger chick has an advantage and may be the sole survivor if food is scarce. While that may seem harsh, it is a common strategy in the natural world which increases the chance that at least one chick will survive.
During Bente’s time in Nome, he thought a high percentage of chicks probably fledged. The raptor surveys were completed when the chicks were one-third to half-way through the nestling phase. He said most nests had two chicks at survey time and were past the more vulnerable early weeks of life when most chick mortality occurs.
Most eggs hatch in the first half of
June. At first, the female stays on the nest to brood the white, downy young, providing warmth when it’s cold and shade when it’s hot. By three weeks, the young have grown enough feathers to maintain their body temperature and are often left alone while both parents hunt.
As the fully-feathered youngsters get close to fledging, they can be seen perched on the edge of the nest flapping and stretching their wings in preparation for their first flight.
Fledging usually happens in late July through mid-August. They stay near the nest for a few weeks and continue to depend on their parents for food for up to a month or perhaps longer.
Once golden eagles establish a breeding territory, they tend to stay there during the nesting season and defend the area from other eagles. However, golden eagles don’t reach sexual maturity for three or four years and young, nonbreeding eagles also summer here, soaring over the entire Seward Peninsula in search of prey and likely prospecting for future breeding sites.
The golden eagle is one of the world’s largest predatory birds. It is remarkable that such large, powerful birds can be so quick and nimble on the wing, but with elegant and efficient flight they are able to maneuver precisely at variable speeds to hunt a variety of prey in various habitats.
Ground squirrels, in summer, and hares, in winter, are preferred foods. However, a dietary study on the Seward Peninsula found that ptarmigan were a larger portion of the diet here than in other populations studied.
Hare numbers cycle dramatically and if they are not available, golden eagles are able to readily switch to hunt ptarmigan, or other seasonal prey such as ducks, seabirds, owls, other birds, newborn reindeer fawns and foxes. Carrion is also scavenged.
By the end of September, most golden eagles have left the Seward Peninsula on their way to wintering areas in southwestern Canada and the western United States.
Rarely, when snowshoe hares are abundant, a golden eagle may winter in the region. Three times over the years, I have seen a single golden eagle in the Nome area in January.
When asked about the impact of our warming climate on golden eagles, Booms replied “Golden eagles are extremely adaptable and do well in many different environments across the globe. Hence, I expect they will adapt much better than other raptor species on the Seward Peninsula. They are not a species I’m too concerned about with climate change.”