A review of the Falconiformes identified 23 species in which obligate and facultative cainism is regular. All species have small clutches and deferred acquisition of adult plumage. Obligate cainists in particular are large, long-lived species characterized by extreme subadult mortality and intense competition for breeding sites. Hence, it can be suggested that early sibling conflict, in the absence of food stress, is the end result of selection for quality (survival) and competitive ability. Cain's domination or killing of Abel insures (1) an increase in Cain's chances of survival through the high-risk, pre-breeding period via improved nestling weight gain, and/or (2) domination of surviving sibs, enhancing Cain's competitive abilities and thereby increasing the probability of achieving breeding status. Only among long-lived species can the benefits of enhanced survival and competitive ability outweigh the major costs of sibling loss.