Reproduction and life cycle of the golden eagle

Mating and egg-laying

Mating and egg-laying timing for golden eagle is variable depending on the locality. Copulation normally lasts 10–20 seconds. Mating seems to occur around 40–46 days before the initial egg-laying. Unusual mating systems of three birds have been recorded in Scotland and Sweden, usually the third being an immature of either sex. These may simply reflect a relatively unstable period of population adjustment. Occasionally, pairs have been recorded copulating outside of the context of fertilization, possibly for pair maintenance and displacement activity. In Japan, a pair was seen mating 46 days before egg laying and in the United States 55 days after egg-laying. The date of egg laying has been directly correlated by the latitude where the pair lives. In the U.S., egg laying can be from anywhere between January and September, though is usually in March or later. In Scotland, egg-laying occurs in March to mid-April. The earliest median laying date in 25 international studies was December 3 in Oman; the latest median date of egg-laying was May 7 in sub-Arctic Alaska. Egg laying starts in the dry season as early as late November around Mali and Niger while the median egg laying date in southwestern Morocco is January 15. In Ethiopia, the estimated range of egg-laying dates ranged from October 24 to January 5. The median egg laying day in Arctic Russia was May 1. In the Kilbuck and Ahklun Mountains area, three pairs had eggs hatched from May 14 to 23, and young fledged from July 8 to August 10 with a median date of July 23 for 11 nests. Two nests on the Kisaralik River still contained young on July 24, and four nests contained young from 16 to 21 July. Clutches have been recorded range in size from 1 to 4 eggs, though two is the norm around the range. Records of single egg clutches are fairly common in Europe, with 3 being rare there but seemingly more common in North America, where up to 12% of clutches include 3 eggs. A 4 egg clutch is considered exceptional. The lowest mean clutch size known from surveys is 1.82 eggs in the Altai Mountains of Russia. A mean of 48 clutches from Algeria and Tunisia showed an average of 1.89 eggs per nests, with a range of 1 to 3. The female laid an average number of eggs of 1.99 in 332 clutches from 8 studies in 5 of the Western United States. The largest mean clutch size across the range was 2.1 in Montana. In the wild, eggs are typically laid at 3 to 5 day intervals, with records in captivity of intervals of up to 7 or 10 days. The eggs vary from all white to white with cinnamon or brown spots and blotches. Scottish eggs averaged 75 by 59 mm (3.0 by 2.3 in) in size and weigh about 145 g (5.1 oz). In Armenia, the average egg measured 76.5 by 59.1 mm (3.01 by 2.33 in) and weighed 123 g (4.3 oz). In North America, eggs average 74.5 by 58 mm (2.93 by 2.28 in), with a range of lengths from 67.5 to 85.7 mm (2.66 to 3.37 in) and a range of width of 49.4 to 63.2 mm (1.94 to 2.49 in). Eggs of golden eagles in California weighed from 113.9 to 176.6 g (4.02 to 6.23 oz), averaging 141.4 g (4.99 oz). The incubation period lasts from 41 to 45 days, averaging about 42.4 days, with previously reported claims of as low as 33 days from North America now known to be erroneous. 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. The female may continue to grab most of their own food in areas where carrion is easily accessible. If made to forage excessively during the incubation stage by a non-attentive mate, the female may abandon the nesting attempt. In Idaho, the female did 84% of the incubation during the day, with the male doing about 16%. At night, the female generally appears to do all of the incubating. In Japan, the female did 90% of the observed incubating.

Early life

The golden eagle chick may be heard from within the egg 15 hours before it begins hatching. After the first chip is broken off of the egg, there is no activity for around 27 hours. After this period, the hatching activity accelerates and the shell is broken apart in 35 hours. The chick is completely free in 37 hours. Upon hatching, the chicks are covered in fluffy white down. One day after hatching, chicks will weigh 105 to 115 g (3.7 to 4.1 oz), with an average of 110.6 g (3.90 oz). In the first 10 days, chicks mainly lie down on the nest substrate. The eagles are capable of preening on their second day but are continually thermoregulated via brooding by their parents until around 20 days. Within 10 days, the hatchlings grow considerably, weighing around 500 g (1.1 lb). Around this age, they also start sitting up more. Around 20 days of age, the chicks generally start standing, which becomes the main position over the course of the next 40 days. 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. After hatching, 80% of food items and 90% of food biomass is captured and brought to the nest by the adult male. The adult male golden eagle perch away from the nest for about 74% of the nestling period, whereas females mainly stay on the nest for about 45 days after the first chick hatches. The voice of nestlings advances from a soft chirp to a disyllabic seeir at around 15 days of age and then to a louder, clearer and conspicuously harsher psaa call given from about 20 days of age to as late as several weeks after fledging. From 10 days to 45 days of age, the nestlings eat much more food and grow considerably. Meal size increases throughout the nesting season, the estimated morsel of flesh fed to the young ranged from 6 mm (0.24 in) at hatching to 15 mm (0.59 in) at fledging. The nestling golden eagles start “mantling” over food at around 10–20 days old: when given a food object, they stand over it, wings partially open, tail fanned and head bowed, covering the food item completely. This is believed to be a competitive behavior as it is seen only in nests with more than one chick. Within a matter of days, the chicks try to defecate over the edge of the nest but are not competent at it until they are around 20 days old. The average daily food consumption for female nestlings tends to be greater than those in male nestlings. The average food consumption of the nestling sexes averages 691 g (24.4 oz) and 381 g (13.4 oz) per day, respectively, with male nestlings weighing about 500 to 600 g (1.1 to 1.3 lb) less than the equivalent-aged females. Despite this, males typically tend to develop sooner and fledge more quickly than the females. The average amount of food brought to the nest daily was notably higher in Idaho and Montana, where an average of 1,417 g (3.124 lb) and 1,470 g (3.24 lb) of prey were brought to the nest, respectively, than in Texas, where an average of 885 g (1.951 lb) was brought. 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. When the first chick is an average of 29 days old, the female first start perching off the nest and at 40 days of age, she stops perching on the nest platform. From 50 days onward, dark brown plumage sprouts from the same sockets as the down and plumage changes become more subtle. The feet go from flesh-colored at hatching to grey to black, thence finally to yellow. When they are around 20 days or so old, as the structure of their wing develops, the nestlings start wing flapping and the frequency and intensity of this behavior increases considerably by 40 days old. Fledging occurs at 66 to 75 days of age in Idaho and 70 to 81 days in Scotland.

Cainism

“Cainism”, as it is sometimes called, or siblicide is inarguably the most controversial and confusing aspect of the golden eagles’ reproductive biology. This is the habitual behavior in the nest of the oldest hatchling to attack and usually kill their young siblings. In fact, this behavior is quite common, not only in large accipitirids but also in unrelated raptorial birds such as skuas and owls. Cainism is frequent, even typical in species of the genus Aquila. The traditionally classified genus can be broken down into two groups: facultative cainists (wherein fewer than 90% of known nests do the oldest nestling attack and kill their younger siblings) and obligate cainists (wherein more than 90% of nests do the older kill the younger siblings). The golden eagle is part of the facultative cainists group, along with the wedge-tailed, eastern imperial, steppe and greater spotted eagles. The obligate cainists are two tropical species, the Verreaux's and the tawny eagle, and one temperate-climate-dwelling species, the lesser spotted-eagle. Other Aquila eagles seem to roughly fall into the tropical species being obligate cainists vs temperate species being facultative cainists categories. Sometimes called “biologically wasteful”, 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. Golden eagles invest much time and effort in bringing up their young; once able to hunt on their own, most golden eagles survive many years, but mortality even among first-born nestlings is much higher, in particular in the first weeks after hatching. This theory is borne out by the fact that the tropical species which are obligate cainists invariably have a longer average nesting period than species which nest in temperate zones. In southwestern Idaho, sibling aggression occurred in all nests with 2-chick broods observed from blinds, and resulted in 1 death in 3 (43%) of 7 broods. In another study, siblicide accounted for 7% of 41 nestling mortalities in southwestern Idaho. 6 (40%) of 15 nestling losses in nests in Central Europe were from cainism. In Scotland, there may be a weak link between food supply and cainism. On Skye, where carrion and rabbits are quite abundant, the younger sibling survives to fledge in about 20% of nests, whereas in the West-Central Highlands, where food is more scarce, the second sibling fledged in only about 4% of nests. However, many nest where food availability was seemingly much higher than what all nestlings would need for food still experience siblicide. The nestlings hatch in approximate 3- to 5-day intervals. If it is a three-egg clutch, the mean estimated weight of the three hatchlings at the time the final egg hatches is 367 g (12.9 oz), 252 g (8.9 oz) and 98 g (3.5 oz), making the largest chick easily dominant and giving the youngest practically no chance of survival. The oldest golden eagle hatchling may start acting aggressively to its younger sibling(s) as soon as it or they hatch. 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. If it is not directly killed, the younger sibling may starve to death, which may be an even more common occurrence. It is apparently common for the younger, weaker siblings to stop begging for food after sibling aggression starts and the parent eagles do not feed the nestlings unless they beg. In some cases, the young nestlings are physically forced from the nest entirely by their older sibling. It is possible that cainism is more common when the older hatchling happens to be a female but, in many cases of males hatching first, they are still larger than the younger siblings and often do dominate and kill them whether male or female. In one nest in Idaho and one in Montana, the oldest sibling was reported to eat their younger siblings, the only verified instances of cannibalism in golden eagles. 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. After the young are about 20 days old, the amount of aggression between siblings (if both survive) decreases and both chicks can usually fledge, though aggression may again increase shortly before fledging.

Fledging and ensuing years

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. The initial flight often includes a short flight on unsteady wings followed by an uncontrolled landing. Young eagles stay within 100 m (330 ft) of the nest in the first few weeks after fledging. They typically have a favored perch where food is brought by the parents and the fledglings only rarely need to take to the wing. 18 to 20 days after first fledging, the young eagles will take their first circling flight but they cannot gain height as efficiently as their parents until approximately 60 days after fledging. Around the time they are 4 months old, the juveniles start to shun the parents attention, even if offered shelter from rain. In Cumbria, young golden eagles were first seen hunting large prey 59 days after fledging and 75 to 85 days after fledging the young were largely independent of parents. In Israel, at 60–70 days old after fledging, the juveniles were still close to nest and quite dependent on parents for food. At the next stage, at around 120 days old, the Israeli juvenile eagles hunting attempts increased and at 160–180 days old they moved further and further away from the nests. The juveniles in Israel first settled over 12 km (7.5 mi) away from the nests. From their first winter until their fourth or fifth winter is the least well-known of the golden eagle’s life. Juveniles disperse widely during their first year, with males remaining closer to the natal area than the more highly exploratory females. In North Dakota, radio-tagged juvenile golden eagles stayed within 5 km (3.1 mi) of the nest for the first 100 days after fledging but then dispersed over 15 km (9.3 mi) in the following 40 days. 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. In the juvenile stage, most Idaho non-breeders stayed within 100 km (62 mi) of their place of hatching, although some birds distributed more than 1,000 km (620 mi) away from their natal range. The movements of first-year eagles from Denali National Park averaged more than 5,500 km (3,400 mi), with surviving individuals migrating south to western Canada and the Western U.S. in autumn then moving back north to western Yukon and Alaska in spring. In Switzerland, juvenile birds traveled an accumulated range of 2,000 to 15,000 km² (770 to 5,790 sq mi) whereas the adults never left their home ranges of 75 to 191 km² (29 to 74 sq mi). A radio-tagged juvenile in Spain travelled a range of more than 16,000 km² (6,200 sq mi) in its first three years of independence, then ultimately settled in a vacant territory 26 km (16 mi) from its hatching place. There is a handful of records of pairs of sub-adult golden eagles (based on their plumage) nesting, sometimes even successfully producing fledglings. The first attempt of nesting by six banded golden eagles in southwestern Idaho occurred when they were from four to seven years of age, with five years appearing to be the average internationally. Once they attempt to nest for the first time, golden eagles will often return to the vicinity of the natal zone, regularly within 7 to 65 km (4.3 to 40.4 mi) of their original nest site, sometimes attacking and even killing older golden eagles pairs if they occupy the area.

Reproductive success

Generally breeding success seems to be greatest where prey is available in abundance. In Central Utah, 57% of eggs successfully hatched out of 87 eggs from 44 clutches. In Idaho, 65% of 282 eggs from 145 clutches successfully hatched, and in Montana, 86% of 28 eggs from 14 clutches hatched. 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. The jackrabbit follows a 10-year cycle where it peaks and crashes. In Idaho, 100% of observed nests produced at least one fledgling when the jackrabbits peaked in the late 1970s through the early 1980s and then at the low point in the mid to late 1980s, the nests produced on average only 0.2 fledglings. Similarly, in central Utah jackrabbit numbers were correlated with average number of young reared by 16 golden eagle pairs. Here the average number of young ranged from 0.56 in 1967 to 1.06 in 1969 to 0.31 in 1973. In Sweden, nesting success rose noticeably in years where the hunting bag (estimated quantity of prey) rose. After viral hemorrhagic pneumonia (VHP) killed off many rabbits in Spain, the average breeding success of golden eagles in Northern Spain dropped from 0.77 in 1982–1989 to 0.38 by 1990–1992. In Belarus, the average brood size is reportedly 1.8 whereas the average number of fledglings is 1.1. As previously mentioned, adverse weather can affect breeding success. In the exceptionally stormy and cold spring of 1984 in Montana, 71% (10 out of 14) studied nests failed. 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. In arid areas, heavier rainfall has the opposite effect and actually improves nesting success. Rainy years in the deserts of Israel, which provide more brown hares and chukars to hunt, were more successful years for breeding. 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. Populations at northern end of range have smaller broods (brood size being about 12% smaller) and produce fewer fledglings (population productivity being about 25% smaller) than those in temperate areas, as has been revealed by contrasting studies of Alaska to Idaho.