Large brood sizes of pied flycatcher, sparrowhawk and goshawk in peak microtine years: support for the mast depression hypothesis.

The mast depression hypothesis (MDH) proposes that cyclic population fluctuations of microtines and other herbivores are an effect of cyclic seed cropping of plants. This is because high seed crops, termed masts, are produced at the expense of chemical defence against herbivores. It has generally been assumed that bird-hunting raptors produce high numbers of offspring when microtine prey are abundant because of reduced competition from generalist predators. However, this may also be caused by higher production of herbivorous insects, and thus insectivorous bird prey, because of lower contents of chemical defence compounds in some plant species, such as bilberry Vaccinium myrtillus and cowberry V. vitis-idaea. In Aust-Agder county, southern Norway, the mean brood size of pied flycatcher Ficedula hypoleuca, sparrowhawk Accipiter nisus and goshawk A. gentilis was higher in peak vole years than in other years. The effect was not due to variation in nest predation, as only successful nesting attempts were included in the analyses. For the pied flycatcher, the annual proportion of large broods (>6 fledglings) was positively correlated with the vole trapping index. No correlation was found between the offspring production of goshawks and the proportion of voles in their diet. During a 3-year light-trapping study of nocturnal moths prior to our study, four moth species whose larvae ate Vaccinium were commonest in the vole peak year. All these results are consistent with the MDH.