Reproduction and life cycle of the golden eagle
This courtship includes undulating displays by both in the pair, with the male bird picking up a piece of rock and dropping it only to enter into a steep dive and catch it in mid-air, repeating the maneuver 3 or more times.
[6] In the wooded peatlands of Sweden and Belarus, a maximum of 5 pairs appear to occur per 1,000 km2 (390 sq mi).
[10][11] Due to the consistency of use by golden eagle pairs, population densities change generally happens only quite gradually.
A massive benefit to cliff nests is that they tend to be largely or entirely inaccessible to mammalian predators on foot (including humans).
[1] In Spain, studies revealed the preferred sites of golden eagle nests were on inaccessible cliffs at a great distance from tracks, roads and villages.
[13] Similarly, in Bulgaria, Italy, Switzerland, France and Yugoslavia, more than 90% of golden eagles nests were located on cliffs.
[18] Rural, arid areas of Europe such as the Iberian peninsula, Provence in France and the Apennines in Italy, fire combined with pastoral activity has maintained suitable nesting sites at relatively low elevations.
[1][28][29][30] Other exceptional nest sites known in North America have included river banks,[31] abandoned gold dredges[32] and electrical transmission towers.
[27] Both heavy rain and excessive heat can potentially kill nestlings, so golden eagles often place their nests to suit the local climate.
[1][40] In a similar situation, little curlews in northeastern Siberia apparently gain some protection from predators by nesting close to golden eagle eyries.
[41] In each case, the natural predators of these animals are just the right size for golden eagle prey, and therefore avoid active eyries.
[1][51] Females do a majority of, but not all, of the incubating and largely attain their own food up to the stage of egg-laying, after which they are typically fed by the male.
[54] If made to forage excessively during the incubation stage by a non-attentive mate, the female may abandon the nesting attempt.
[62] The whitish down continues until around 25 days of age, at which point it is gradually replaced by dark contour feathers that eclipse the down and the birds attain a general piebald appearance.
[60][66][67] For the first 30 days or so, the nestlings are fully dependent on their parents to feed them but after that period, they start standing around the edge of the nest and practice food tearing.
[1][60] “Cainism”, as it is sometimes called, or siblicide is inarguably the most controversial and confusing aspect of the golden eagles’ reproductive biology.
In fact, this behavior is quite common, not only in large accipitirids but also in unrelated raptorial birds such as skuas and owls.
[51][69][70] Sometimes called “biologically wasteful”,[71] this strategy is most commonly explained as useful for the species because it makes the parents' workload manageable even when food is scarce, while providing a reserve chick in case the first-born dies soon after hatching.
Within the first two days, this often escalates into “bill-stabbing” wherein the younger sibling is jabbed around their neck or the middle of their body until a gaping, fatal wound is created.
[2][75] Although the brooding mothers, otherwise famous for the high level of their parental care, is fully aware of the sibling aggression, in no raptorial bird species are they known to intervene when cainism occurs.
[1] The first attempted flight departure can be abrupt, with the young jumping off and using a series of short, stiff wing-beats to glide downward or being blown out of nest while wing-flapping.
They typically have a favored perch where food is brought by the parents and the fledglings only rarely need to take to the wing.
Juveniles disperse widely during their first year, with males remaining closer to the natal area than the more highly exploratory females.
The study in North Dakota focused on juveniles from six different nests which successfully produced two fledglings and the behavior of the sibling-pairs was surprisingly gregarious as they flew together, perched together and mutually preened for months after independence.
[83] There is a handful of records of pairs of sub-adult golden eagles (based on their plumage) nesting, sometimes even successfully producing fledglings.
[2][89] In Scotland, the highest breeding success of golden eagles was in the Eastern Highlands where heather moorland still abundant, bearing plentiful red grouse and mountain hare.
[94] A long period of exceptionally rainy, cold springs in Scotland reportedly resulted in a 25% reduction of fledging pairs being produced.
Rather than directly killing the nestlings, stormy, wet weather probably causes the most harm to productivity due to the hampering of the parents' ability to hunt.
[96] While siblicide is estimated to claim about 80% of second hatchlings among golden eagles, in some parts of the range, the survival rate in successful nests is higher.
In ideal habitats in North America (Northwestern United States and Alaska), 38 to 56% of nests produce a second fledging.
