Offspring quality and the evolution of cainism

Obligate siblicide, known as ‘cainism’ in large raptors, is a taxonomically widespread avian phenomenon that remains inexplicable as a simple consequence of food stress: two young can be raised to independence in experimentally manipulated nests, and food supplements do not decrease sibling aggression. 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. Facultative cainists. which in more than 10% of cases raise more than one young (despite aggression and sibling hierarchies), not only lay larger, more variable clutches, but on average attain adult plumage earlier than obligate cainists. Their shorter lives and higher population turnovers are consistent with their less extreme siblicidal tendencies. Similar life-history traits and cainistic habits in other avian orders parallel those in the Falconiformes, indicating several independent evolutionary pathways to cainism. Retention of the second egg by obligate cainists, usually explained as insurance against failure, may instead allow parents adaptively to track population stability. Thus when breeding places are numerous (habitat saturation and competition low), parents laying two eggs and rearing two young may achieve greater fitness than single-young parents. When populations become saturated (competition high), selection should favour high-quality, competitive young and levels of siblicide should increase. Aproximate mechanism is proposed linking population saturation with the incidence of cainism, based on demonstrable population characteristics found in several long-lived species.