MANEN W VAN (2001) Influence of weather circumstances on behaviour and hunting success of wintering Long-eared Owls Asio otus. LIMOSA 74 (3): 81-86.

In the winter 1991/92 I studied a wintering group of Long-eared Owls. They spent the day in a group of Scots Pines Pinus sylvestris in which they were well visible and in an adjacent group of Spruce Picea abies in which they could not be counted. The number owls in pines (Fig. 1) showed some irregular dips in the course of winter which are probably caused by owls moving into the spruces. At calm days they preferred to roost in pines (Fig. 2), because the open crowns allowed them to catch some sun. In Long-eared Owls solar radiation causes saving of energy il ambient temperatures are below 10 °C (Wijnandts 1984). On windy days however, the owls moved into the protective spruces to reduce energy loss by wind. Roosting behaviour was not influenced by temperature or hours of precipitation.
      By analyzing all pellets collected under the roosting trees I estimated the species composition and amount of prey eaten per night. Long-eared Owls seemed to hunt most profitably at a wind speed of 4.1 m/s (Fig. 3). Probably their most efficient hunting method resembles that of a harrier, which means that it is easier to hunt in slight headwind. On the other hand, mice and voles may avoid predation by being less active during quiet nights, when they are more easily detected. During strong winds the owls probably have difficulties in detecting prey.
      The proportion of Short-tailed Vole Microtus arva lis in the diet decreased with increasing wind speed whereas the proportion of Wood Mouse Apodemus sylvestris and Bank Vole Clethrionomys glareolus initially increased with wind speed and only dropped at a very high wind speed (Fig. 4). This pattern indicates that in light winds the owls hunted in open areas, but with increasing wind speed they increasingly hunted in sheltered areas. At every wind speed birds were eaten, but only during very strong wind they became the most important prey. The time at witch the owls departed from their roost was highly correlated with the amount of cloud cover (Fig. 5). On heavily overcast evenings they left earlier. No relationship was found with wind speed, nor with the amount of food that was consumed in the previous night. Long-eared Owl Asio otus

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