Numerical response of common buzzards and predation rate of main and alternative prey under fluctuating food conditions.

We studied the numerical response of a population of the common buzzard Buteo buteo to the fluctuations of its main prey (voles) and alternative prey (forest grouse) in western Finland between 1979 and 1996. Populations of main prey fluctuated in a cyclic manner with three years between peak densities. The nesting success of buzzards averaged 1.7 fledglings per nesting attempt and the brood size averaged 2.2 fledglings per brood. The nesting rate (no. of active nests per number of occupied territories) and the productivity rate (no. of chicks for all territories) positively correlated with the abundance of Microtus voles in the current spring but not with the abundance of grouse. The Alternative Prey Hypothesis (APH) predicts that, in the years when the main prey species decline, generalist predators can shift their diet to alternative prey and thus cause its decline. The Shared Predation Hypothesis (SPH) states that all important prey species, including alternative prey, are under high hunting pressure when the density of predators is high. The predation rate (the combination of numerical response and previously studied functional response) of Microtus voles by buzzards was positively correlated with the densities of these voles in the current spring, whereas the predation rate of grouse tended to peak one year after peak densities of Microtus voles. Therefore, our results appear to support APH rather than SPH and indicate that predation by buzzards may dampen population cycles of main prey (voles) but amplify population fluctuations of grouse.

Reif, V., Jungell, S., Korpimaki, E., Tornberg, R. & Mykra, S. 2004, Ann. Zool. Fennici 41: 599-607