Nest size affects clutch size and the start of incubation in magpies: an experimental study

Nest size has been suggested to be a sexually selected trait indicating parental ability of both males and females. To test whether a female's reproductive decisions (e.g., clutch size and starting incubation) change in relation to experimental manipulation of nest size, as would be predicted if nest size is a sexually selected signal reflecting the male's parental quality, we manipulated nest size in a population of monogamous magpies before laying by adding or removing about 20 cm of large sticks in the roof of magpie nests. On the one hand, we found that clutch size of reduced nests was smaller than that of control or enlarged nests. Moreover, clutch size was significantly related to nest size after manipulation, which indicates that females adjust clutch size to the final size of the nest, nest size thereby being a good candidate for a sexually selected trait. On the other hand, number of eggs hatched during the first day is hypothesized to be related to the expected available resources during nestling growth, and subsequent nestlings hatched are likely to die due to brood reduction if resources are not sufficient to raise well-developed nestlings. Nest size is hypothesized to inform females about a male's willingness to invest in reproduction, and we found that in broods of experimentally reduced nests, females started to incubate earlier in the laying sequence than they did in broods of control or enlarged nests. Moreover, in experimentally reduced nests, fewer nestlings hatched during the first day, and the difference in body mass between the first and the fourth nestling hatched increased. This result is in accordance with the hypothesis that the female's decision of when to start incubation in the laying sequence is mediated by nest size, a sexually selected trait signaling parental quality. We discuss alternative explanations for the results such as the possibility that nests of different treatments may differ in their thermoregulation properties or in their protection against predators.