Male and female imperial eagles form monogamous pairs at around four years old and then stay together for life. They build a large nest, known as an eyrie, from sticks, at the top of a tall tree (2), and will return to this and a couple of other nests in rotation every year, making repairs as necessary (6). During the spring, the female lays between two and four eggs, which are incubated for 43 days by both parents, hatching from the end of May to the middle of June. The smallest hatchling is usually pecked or starved to death by its older, stronger sibling, which claims more of the adults' attention. The surviving nestling will learn to fly at around two months, but will stay at the nest for another few weeks, being fed by the female until it can hunt (2). The imperial eagle usually hunts alone, targeting small mammals (mainly ground squirrels known as susliks (Spermophilus citellus)), reptiles, birds and carrion (2). They have excellent eyesight for spotting prey whilst gliding, but they may also steal the catch of other birds of prey, sometimes obtaining the majority of their food this way (6). Whilst each bird begins its migratory journey alone, imperial eagles often congregate into loose flocks of ten or more to soar on level wings, covering up to 8,000 kilometres in six weeks (2).