Influence of season and availability of forest rodents on diet composition of tawny owl Strix aluco inhabiting field-forest mosaic in central Poland

The aim of the study was to characterize the diet composition of tawny owl in relation to environmental variables: season and availability of small rodents. Study area was located in central Poland, in Lodz voivodship, in the area of Rogow Forest District. It comprised of a mosaic of forests and arable lands. The study was done in the years 2003-2010. Pellets were collected at least twice a year in 18 tawny owl territories. Material collected between April and September was categorised as coming from spring-summer, while this from October-March period as autumn-winter season. Simultaneously, in the years 2004-2007, changes in the number of small rodents were monitored by live-trapping. Standard procedures of pellet analyses were used to identify 1926 prey items. Generally, diet composition was dominated by small rodents. Yet, their share was significantly higher in autumn-winter season (71.3 vs. 57.3%). Among rodents, yellow-necked mice, common voles and bank voles were caught most often. Soricomorphs accounted for 3% of prey items in both seasons on average. Birds formed approximately 15% of prey items and their share was comparable in warm and cold half year. On the other hand, in a warmer period owls preyed on invertebrates more often (23.0 vs. 7.5%). A breadth niche was wider in a warm (D=4.01) half year than in cold one (D=3.36). When a peak of rodent number was recorded, owls preyed on bank voles twice as often as compared to other years (increase from 6.4 to 11% of prey items), while an increase in the consumption of yellow-necked mouse was lower (from 27.5 to 34.2%). Tawny owls preferred yellow-necked mice and bank voles were avoided. This study showed that in the Rogow Forest tawny owl is an opportunistic predator. However, composition of its diet does not absolutely reflect the structure of assemblage of small mammals as certain species are preferred.