Brief report Sex-specific diet analysis of the Eurasian Eagle Owl in Finland Heimo Mikkola* & Risto Tornberg H. Mikkola, Department of Biology, University of Eastern Finland, P.O.Box 1627, FI- 70211 Kuopio, Finland. * Corresponding author’s e-mail: heimomikkola@yahoo.co.uk R. Tornberg, Department of Biology, P.O.Box 3000, FI-90014 University of Oulu, Fin- land Received 28 March 2014, accepted 17 June 2014 1. Introduction Birds of prey typically show reversed sexual size dimorphism (RSD), with females clearly larger than males, for what purpose has aroused a pleth- ora of hypothesis to explain it (see Krüger 2005). One of the earliest explanations has been avoid- ance of competition between the mates (Temeles 1985). Though avoidance of competition does not explain the reversed nature of the sizes of the sexes it may be one mechanism to maintain it (Sunde et al. 2003, Krüger 2005). The Eurasian Eagle Owl (Bubo bubo), hereafter termed Eagle Owl, is the largest owl in the world, a typical top-predator be- ing able to kill smaller birds of prey and owls (Mikkola 1983, 2013, Tornberg & Colpaert 2001). Its breeding season diet is well-known and, based on pellet analysis and nest site prey remains, shows very broad variation (Mikkola 1983, Sulkava et al. 2008). The Eagle Owl, mainly de- pendent on small mammals as its staple food dur- ing the breeding season (Korpimäki et al. 1990, Sulkava et al. 2008), exhibits high RSD (Mikkola 1982). However, sexual differences in diet are not ex- pressed by pellet and food remains analysis, which might arise from the fact that in Eagle Owls, it is the smaller male that is mainly responsible for prey deliveries during the breeding season. Based on specimens found in good condition at the Zoologi- cal museum of University of Oulu, Finland, fe- male owls from Finland had an average weight of 2760 g (N = 50), while male weights averaged 2200 g (N = 35). The RSD index of the Eagle Owls is 9.8 (calculated as in Amadon (1943) and Earhart & Johnson (1970) by using the cube root of body mass to compare the indices of linear measure- ments). This is the third highest value of all Euro- pean owls (Mikkola 1982). It seems logical to hy- pothesize that the heavier females may reduce in- traspecific competition for food with males by tak- ing larger prey than their smaller mates. However, this is not easy to verify as it is not possible to dif- ferentiate between pellets and food remains from males and females. In addition, a Russian study has shown that male Eagle Owls mainly bring larger prey to the nest than they would normally eat themselves, making it even more difficult to identify sex-related diet differences from pellets and food remains at the nest (Ekimov 2009, see also Sonerud 1992 and Sonerud et al. 2013, for theoretical point of view). In this study we used the stomach contents of Eagle Owls (primarily 1 st winter birds) made avail- Ornis Fennica 91:195–200. 2014