WIND-SPEED AS A DETERMINANT OF KLEPTOPARASITISM BY EURASIAN KESTREL FALCO-TINNUNCULUS ON SHORT-EARED OWL ASIO-FLAMMEUS

Twenty-five kleptoparasitic attempts by Kestrels on Short-eared Owls w ere recorded in 16 afternoons of observation (70 hours) during the win ter of 1996-97 in western France. Nine attempts were successful. The d aily hunting yield of Kestrels is related to the time spent flight-hun ting, an energetically costly hunting technique. Medium to strong wind s allow Kestrels to use flight-hunting at a lower cost, hence increasi ng the rate of net energy intake. I found that kleptoparasitism occurr ed more frequently when wind speed was unfavourable for flight-hunting (i.e. weak winds). Food theft was more successful when done by a pair ; the female always initiated the attacks. One stolen prey per day was estimated to compensate for 50% of the potential reduction in hunting yield caused by the lesser time spent flight-hunting in poor wind con ditions.