DO NOMADIC AVIAN PREDATORS SYNCHRONIZE POPULATION FLUCTUATIONS OF SMALL MAMMALS - A FIELD EXPERIMENT

Tree-to-five-year population oscillations of northern small rodents ar e usually synchronous over hundreds of square kilometers. This regiona l synchrony could be due to similarity in climatic factors, or due to nomadic predators reducing the patches of high prey density close to t he average density of a larger area. We estimated avian predator and s mall rodent densities in 4-5 predator reduction and 4-5 control areas (c. 3 km(2) each) during 1989-1992 in western Finland. We studied whet her nomadic avian predators concentrate at high prey density areas, an d whether this decreases spatial variation in prey density. The yearly mean number of avian predator breeding territories was 0.2-1.0 in red uction areas and 3.0-8.2 in central areas. Hunting birds of prey conce ntrated in high prey density areas after their breeding season (August ), but not necessarily during the breeding season (April to June), whe n they were constrained to hunt in vicinity of the nest. The experimen tal reduction of breeding avian predators increased variation in prey density among areas but not within areas. The difference in variation between raptor reduction and control areas was largest in the late bre eding season of birds of prey, and decreased rapidly after the breedin g season. These results appeared to support the hypothesis that the ge ographic synchrony of population cycles in small mammals may be driven by nomadic predators concentrating in high prey density areas. Predat ion and climatic factors apparently are complementary, rather than exc lusive, factors in contributing to the synchrony.