Asynchronous Hatching in the Pied Flycatcher: An Experiment
1991
Amundsen, Trond | Slagsvold, Tore
David Lack's “brood reduction hypothesis” to explain asynchronous hatching has recently been tested experimentally on several altrical species of birds. However, for most studies it is not known whether a food shortage occurred or not. We induced food shortage by removing the male parent shortly after hatching, in Pied Flycatcher (Ficedula hypoleuca) broods with experimentally reduced and increased asynchrony. The broods were adjusted to an initial size of five. Broods with reduced and increased asynchrony showed a similar degree of partial mortality, fledging on average 3.3 and 3.5 young, respectively. In broods with both parents present and a normal level of asynchrony, all the young fledged. Mean fledgling body masses in reduced (12.6 g) and increased asynchronous (13.1 g) uniparental broods were also similar, and significantly lower than that of biparental broods (14.6 g). Pronounced nestling size hierarchies developed in both types of experimental broods, and female body mass losses during the nestling period were similar. The results demonstrate that, in the Pied Flycatcher, asynchronous hatching is not necessary for the achievement of brood reduction. The differences between broods with reduced and increased asynchrony in numbers and body masses of fledged young were not statistically significant, but were in the direction predicted by the brood reduction hypothesis. The results are inconsistent with Lack's original formulation of the hypothesis, but not with a recent life history modification, which states that asynchrony increases parental efficiency rather than within—season nestling survival.
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