Effects of agriculture development on vole dynamics and conservation of Montagu's harrier in western French wetlands

Nesting populations of Montagu's harrier ( Circus pygargus) are declining in most parts of Europe; in France, western marshes remain the most important nesting sites in terms of breeding pairs. In this open field landscape dominated by grasslands, the common vole ( Microtus arvalis) displays regular population outbreaks and constitutes a favourite prey of this raptor. Twelve years of field data indicate significant variations in nesting population size and young harriers produced, which correlate with yearly differences in vole densities. Up to 10 years ago, these marshes were traditionally used as extensive pastures but recent agricultural changes have resulted in almost 50% of the pastures being converted to drained agricultural production, as already observed in many localities of this region next to our study area. Our data, together with previous data collected from 1968 in this region, demonstrate that agricultural changes have resulted in a decrease of frequency and intensity of vole population peaks. A summer density of 100 voles/ha appears as a threshold value to support a good breeding success of harriers. These modifications of the vole fluctuation pattern suggest that nesting populations of Montagu's harrier from western French marshes could be endangered in future under these current trends in agricultural changes.