The reproduction habits of Nazca boobies are unusual, predictable and cruel. The lack of predators allows the seabirds to nest on barren ground near cliff edges. The mature female lays two eggs a year. The season varies by island in the Galápagos. At Genovesa Island, the timing is August through November. The parents take turns incubating the eggs with their feet for 40 days. The birthrate is low. Only 25% to 50% of the eggs survive. The first chick arrives four to ten days before the other. When the second one hatches, it is always rejected by the elder and the parents, leaving it to die from starvation. The behavior is called obligate siblicide. Ornithologists believe the seemingly barbaric practice evolved because of the historic shortage of food.