Hatching asynchrony as a parental reproductive strategy in birds: a review of causes and consequences

Bird parents may choose among various breeding strategies. From an evolutionary point of view the outcome of their choice is believed to provide the highest reproductive success in a given environment. However, some of the mechanisms underlying parental reproductive decisions are still poorly understood, for example the phenomenon of hatching asynchrony. Resultant age and size hierarchy among nest mates often lead to competitive disadvantages, starvation and death of last-hatched siblings. Although synchronised broods generally result in higher fledging success, altricial bird species typically hatch asynchronously. Various hypotheses were proposed and tested to explain this strategy. A quarter of a century has passed since the last review of research on hatching asynchrony. In the meantime the number of studies investigating the issue has considerably grown, same as the array of hypotheses. Our aim is to present a comprehensive synthesis of the research on 20 hypotheses presented until recently as well as to indicate the blind spots in the reviewed area—9 out of 20 formulated hypotheses still lack any experimental approach that would confirm or reject the assumptions.