Delayed numerical response of goshawks to population fluctuations of forest grouse

Delayed density-dependent mortality induced by delayed numerical response of predators can drive prey populations to fluctuate in high-amplitude cycles. We studied numerical response of goshawks Accipiter gentilis to varying densities of their main prey (forest grouse) in western Finland during 1979–1996. Occupancy rate of goshawk territories tracked grouse numbers with a two year lag. Occupancy rate of goshawk territories and pooled number of adult and young goshawks correlated negatively with a 1–2 year lag to the chick production of grouse. Goshawk to grouse ratio was negatively related to grouse density. This suggests that goshawk predation on grouse is inversely dependent on grouse density. We conclude that in northern Europe with few alternative preys, goshawk predation might contribute to the generation of multiannual cycles of forest grouse. This should be tested experimentally.