Timing of vole hunting in aerial predators: RAPTOR GROUP RUG/RIJP*

In a short but intense field investigation, surface activity of common voles in a cropped lucerne field was assayed by live-trapping while, simultaneously, hunting activity and yield of three species of raptors were recorded by continuous observation. Pronounced short-term rhythms in trappability of the vole population ran parallel with fluctuations in yield per hour of flight-hunt of hen harriers and kestrels. These raptors, as well as Rough-legged buzzards, hunted more at times of increased vole surface activity; hen harriers saved c. 15 hours of flight-hunt per day by such temporal adjustment, corresponding to about 12% of their daily energy intake. Voles suffered a predation rate of an estimated 0 2% per day; under such heavy predation and temporal concentration of raptor hunting at times of increased vole activity, surface feeding in synchrony with the vole majority was associated with increased risk of predation.