Asynchronous Hatching in Passerine Birds: Influence of Hatching Failure and Brood Reduction

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Abstract
Asynchronous hatching, which is widespread among altricial species of birds, has primarily been explained as a mechanism by which the parents can adjust brood size to food availability. This brood reduction hypothesis is analysed, using data obtained from a study of five passerine bird species in Norway, 1978-1981. For each species the total number of young fledged in relation to the number of eggs laid was about 20% lower for nests in which the hatching spread was increased experimentally (to about four days), than in the control nests. This was due to an increased rate of hatching failure and to increased nestling mortality. An increase in hatching losses was very pronounced in the case of the open-nesting species. It is therefore remarkable that these birds in general hatch their eggs relatively asynchronously. Contrary to a prediction from the brood reduction hypothesis, the breeding success of the Great Tit Parus major was reduced to a greater degree in spruce-alder woodland than in rich deciduous woodland when hatching spread was experimentally increased. This was also the case with the breeding success of the Pied Flycatcher Ficedula hypoleuca.