Aquila chrysaetos
Golden Eagles usually mate for life. They build several eyries within their territory and use them alternately for several years. These nests consist of heavy tree branches, or nooks in cliffs, upholstered with grass when in use. Old eyries may be 2 meters (6.6 ft) in diameter and 1 meter (3.3 ft) in height, as the eagles repair their nests whenever necessary and enlarge them during each use. If the eyrie is situated on a tree, supporting tree branches may break because of the weight of the nest. The female lays one to four (usually two) eggs between January and September (depending on the locality). The eggs vary from all white to white with cinnamon or brown spots and blotches. They start incubation immediately after the first egg is laid, and after 40 to 45 days the young hatch. They are covered in fluffy white down and are fed for fifty days before they are able to make their first flight attempts and eat on their own. In most cases only the older chick survives, while the younger one dies without leaving the eyrie. This is due to the older chick having a few days' advantage in growth and consequently winning most squabbles for food. This strategy is 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.
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