Sibling aggression, hatching asynchrony, and nestling mortality in the black kite (Milvus migrans)

In siblicidal species, hatching asynchrony could act to reduce sibling riva lry or promote the death of last-hatched chicks. The pattern of hatching as ynchrony was experimentally altered in the black kite Milvus migrans. Hatch ing asynchrony in control broods was intermediate between those of experime ntally synchronised and asynchronised broods. Sibling aggression and wounds on the chicks were more commonly observed early in the nestling period and in synchronous nests. Serious injuries were observed on last-hatched chick s in asynchronous nests, as were observations of intimidated or crushed chi cks. Sibling aggression was related to food abundance, but some chicks died at an early age in nests with abundant food (cainism). Cainism was more co mmonly found in asynchronous nests. For species with facultative siblicide, moderate hatching asynchrony could be a compromise between reducing siblin g rivalry and avoiding large size differences between sibs that would resul t in cainism. Female black kites preferentially fed the smallest chicks and exhibited behaviours to reduce sibling aggression, contrary to observation s in other siblicidal species. In a highly opportunistic forager such as th e black kite, a strategy may exist to protract the life of all the chicks i n the brood, waiting for unpredictable situations of food overabundance. Th is would induce the appearance of a parent-offspring conflict over brood re duction, reflected in the existence of a possible anticipated response by s ome of the chicks (cainism) and in the appearance of special behaviours by the parents to selectively feed smaller chicks or reduce sibling aggression . In this facultatively siblicidal species, cainism does not seem to be the final stage of an evolutionary trend favouring the raising of high-quality chicks, but a manifestation of a parent-offspring conflict over brood size .