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Wesołowski T. 2004. The origin of parental care in birds - a reassessment. Behavioral Ecology 15: 520-523.

Males and females care jointly for young in over 90% of avian species, what makes birds unique. How this peculiar pattern of parental care system has evolved is still not fully understood. Two major evolutionary scenarios have been proposed 1) changes in the mode of parental care in some ordinary reptiles, achievement of advanced biparental care (consisting of egg guarding, incubation, as well as protecting and escorting of young) by them, constituted the first major event in avian evolution. The ability to fly came later, without a tight relationship to the parental care mode; 2) active flight evolved for reasons independent of reproduction, and parental care has evolved as a response to it, within a framework of limitations set by adaptations to aerial locomotion. The evolution of birds proceeded initially through stages without parental care (main adaptations to flight appeared then) but with increasing investment in eggs, resulting in the appearance of sequential ovulation and of very large eggs producing young able to fly shortly after hatching. The parental care appeared only at that stage, and that it was initially unaided male care. Biparental care evolved later from unaided male care. The strength and validity of the main paleontological, phylogenetic and cost/benefit arguments put forward to differentiate between the two scenarios are critically evaluated and a set of minimum requirements which any comprehensive theory of the origin of parental care should fulfil.