Evol Ecol Res 1: 753-768 (1999)     Full PDF if your library subscribes.

Variation in egg mass in the Pied Flycatcher, Ficedula hypoleuca: An experimental test of the brood survival and brood reduction hypotheses

Lars Hillström *

Department of Zoology, Uppsala University, Villavägen 9, S-752 36 Uppsala, Sweden

Address all correspondence to Lars Hillström, Department of Animal Ecology, Evolutionary Biology Centre, Norbyvägen 18D, S-752 36 Uppsala, Sweden.
e-mail: lars.hillstrom@zoologi.uu.se


Variation in egg mass is a characteristic feature among birds and varies both between and within species. It has been suggested that, in altricial birds that hatch their eggs asynchronously, two tactics regarding intra-clutch variation in egg mass could have evolved. Which of these two strategies is used depends on factors important for nestling survival – availability of food and nest predation. If when laying eggs parents can predict availability of food during nestling rearing, and if low food supplies are predicted, they should adopt a ‘brood reduction strategy’; that is, invest less in the last eggs laid to minimize any wasted energy if all young cannot be raised. In contrast, if prospects are good for raising all young, parents should instead adopt a ‘brood survival strategy’; that is, invest relatively more in the last eggs laid, as the chances of raising the last chicks to be hatched are better during good years. However, if nest predation is more important for nestling survival than food availability, and parents begin incubating before clutch completion to reduce nest predation (on eggs), then parents should also use a ‘brood survival strategy’. I studied variation in egg mass in a population of Pied Flycatchers and found that there was a significant increase in egg mass with laying order. In an experiment in which last eggs to be laid were swapped between nests to increase or reduce the egg mass of the last egg laid, there was a positive effect of egg size on hatching mass and on nestling growth rate during the early nestling phase. Thus, nestlings hatching asynchronously and ‘originating’ from an experimentally larger egg, were compensated in their later hatching by a higher elevation of the slope of the growth curve compared with their siblings during the first part of the nestling phase. I also found that egg mass was positively correlated with wet yolk and wet albumen, as well as with energy content of the yolk and total energy content of the egg. Thus, Pied Flycatchers appear to use a ‘brood survival strategy’; that is, they lay a large last egg, which contains more nutrients, to compensate for the late hatching of the last egg. It has also been shown for this species that, in a population breeding in natural holes, predation during the egg stage is the most important mortality factor.

Keywords: brood reduction, Ficedula hypoleuca, hatching asynchrony, variation in egg mass.

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