Originally proposed by Dorward (1962), the insurance egg hypothesis (IEH) has quickly become the most widely supported explanation for avian siblicide as well as the overproduction of eggs in siblicidal birds. The IEH states that the extra egg(s) produced by the parent serves as an "insurance policy" in the case of the failure of the first egg (either it did not hatch or the chick died soon after hatching). When both eggs hatch successfully, the second chick, or B-chick, is known as the marginal offspring; otherwise stated, it is marginal in the sense that it can add to or subtract from the evolutionary success of its family members. It can increase reproductive and evolutionary success in two primary ways. Firstly, it represents an extra unit of parental success if it survives along with its siblings.