Three-year periodicity in historical raptor-persecution data: an indication of vole cycles?

Abstract

Reflections of three- to four-year cycles in vole abundance have been observed in the breeding performance and abundance of raptors in Estonia at least since the 1970s. To check whether such link could be used to reconstruct earlier vole cycles, too, I analysed two geographically distinct samples of reports on systematic raptor persecution in 1890-1908. These samples showed three- year periodicity and coincided in their years of minimum although vole-eating species formed only a small part of the material. The numbers of different taxa (diurnal raptors vs owls; Buteo buteo vs Accipiter nisus) were positively correlated in the material, indicating interspecific synchronity in the fluctuations. Probably the observed pattern reveals that, similarly to the recent decades, the productivity of raptors fluctuated in response to three-year vole cycles in Estonia already at the end of the 19th century. These reconstructions can explain some known historical irruptions of raptors and indicate an early general decline of diurnal raptors at least in southern Estonia.