Ornis Fennica 91:195–200. 2014 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: [email protected] 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 the smaller male that is mainly responsible for prey deliveries during the breeding season. Based on Birds of prey typically show reversed sexual size specimens found in good condition at the Zoologi- dimorphism (RSD), with females clearly larger cal museum of University of Oulu, Finland, fe- than males, for what purpose has aroused a pleth- male owls from Finland had an average weight of ora of hypothesis to explain it (see Krüger 2005). 2760 g (N = 50), while male weights averaged One of the earliest explanations has been avoid- 2200 g (N = 35). The RSD index of the Eagle Owls ance of competition between the mates (Temeles is 9.8 (calculated as in Amadon (1943) and Earhart 1985). Though avoidance of competition does not & Johnson (1970) by using the cube root of body explain the reversed nature of the sizes of the sexes mass to compare the indices of linear measure- it may be one mechanism to maintain it (Sunde et ments). This is the third highest value of all Euro- al. 2003, Krüger 2005). The Eurasian Eagle Owl pean owls (Mikkola 1982). It seems logical to hy- (Bubo bubo), hereafter termed Eagle Owl, is the pothesize that the heavier females may reduce in- largest owl in the world, a typical top-predator be- traspecific competition for food with males by tak- ing able to kill smaller birds of prey and owls ing larger prey than their smaller mates. However, (Mikkola 1983, 2013, Tornberg & Colpaert 2001). this is not easy to verify as it is not possible to dif- Its breeding season diet is well-known and, based ferentiate between pellets and food remains from on pellet analysis and nest site prey remains, males and females. In addition, a Russian study shows very broad variation (Mikkola 1983, has shown that male Eagle Owls mainly bring Sulkava et al. 2008). The Eagle Owl, mainly de- larger prey to the nest than they would normally pendent on small mammals as its staple food dur- eat themselves, making it even more difficult to ing the breeding season (Korpimäki et al. 1990, identify sex-related diet differences from pellets Sulkava et al. 2008), exhibits high RSD (Mikkola and food remains at the nest (Ekimov 2009, see 1982). also Sonerud 1992 and Sonerud et al. 2013, for However, sexual differences in diet are not ex- theoretical point of view). pressed by pellet and food remains analysis, which In this study we used the stomach contents of might arise from the fact that in Eagle Owls, it is Eagle Owls (primarily 1st winter birds) made avail- 196 ORNIS FENNICA Vol. 91, 2014 Fig.1. Number of dead Eagle Owls sent to the Zoological museum of University of Oulu (grey bars) and correspond- ing vole abundance in- dex (solid line) near city of Oulu. First two peak years of voles (before the vole monitoring started at the University of Oulu) are presented by arrows according to Mikkola 1981. able from museums or taxidermists, which is a re- Prey items were identified based on bones, liable method to study between sex differences in hairs and feathers using reference material of the diet provided that the owls have been sexed inter- Zoological museum. Small mammals were identi- nally (Mikkola 1971). We expect that based on a fied into species level by jaw and tooth morphol- remarkable size difference between the sexes a ogy according to Siivonen & Sulkava (1994). corresponding difference in the prey size should For the prey items, we used average weights exist. given by: (a) Siivonen & Sulkava (1994) and Jensen (1994) for mammals; and (b) von Haart- man et al. (1963–1972) for birds. For birds show- 2. Material and methods ing remarkable sexual dimorphism we used spe- cial weight for the sex in question. Differences in The contents of the stomachs of 66 Finnish Eagle diet between sexes were tested by ¤2-tests in a 2 × 2 Owls, found dead along roads, having flown into contingency table. We applied Yates continuity power lines and having been killed and confis- correction, which is recommended when data is cated after illegal hunting, were analysed. A total divided into 2 categories or frequency within some of 85 prey items were identified from 42 females of cells remains low (Ranta et al. 1989). Diet width and 48 from 24 males. The samples were collected was calculated by using Levins’ index (Levins S 2 by taxidermist Pentti Alaja, and the University of 1968) B =1/ Pi ,inwhichPi is the proportion of Oulu from 1931 to 2006. Around 47% of the owls the ith prey or prey group. were found dead in autumn (September–Novem- ber), 14% in mid-winter (December–February) and rest 40% during the breeding season (March– 3. Results August). Of those owls for which cause of death was known (N = 51), 43% died in collision with The most common prey for both male and female power lines, 33% in traffic accidents, 10% in colli- owls was Field vole (Microtus agrestis) by num- sion with unknown obstacles, 4% were shot and bers (31 and 29% respectively), but this vole rep- for 2% cause of death was unknown. More than resented only 3% of the female and 4% of the male half (55%) of our age determined study birds were diet biomass. Somewhat surprisingly, the Harvest juveniles (N = 38). Owls were found, most often mouse (Micromys minutus) was the second most south of the city of Oulu (25°30’ E, 65°00’ N) at common prey by numbers, making 22% of the diet less than 200 km distance. We classified each of females and 29% of the males. Due to its small sample according to whether it was found in a size (avg. 7 g), the weight percentages were only good or poor vole year based on vole trappings 0.4% for the females and 0.7% for the males (Table mainly by staff of Zoological museum of Univer- 1). By pooling small mammals together, the differ- sity of Oulu and the Forest Research Institute ence between females and males was not statisti- (METLA) (see Fig. 1). According to our classifi- cally significant (¤2 = 0.939, df = 1, p >0.1).Both cation, 35 owls originated from a poor vole year female and male mainly ate Mountain hares and 31 from a good vole year (Fig. 1). (Lepus timidus), which formed over 56% of the Mikkola & Tornberg: Sex-specific diet in the Eurasian Eagle Owl 197 Table 1. Sexual differences in the diet of Eurasian Eagle Owls in Finland based on 66 stomach contents (42 females and 24 males). Average weights calculated from Siivonen & Sulkava (1994) and Jensen (1994) for mammals, and from von Haartman et al. (1963–1972) for birds. Females Males Total Prey item Weight g % mass % N % mass % N % mass % N Micromys minutus 7 0.4 22.4 0.7 29.2 0.5 24.8 Myodes glareolus 24 0.2 3.5 0.5 6.3 0.3 4.5 Small passerines 25 0.1 1.2 0.1 0.8 Cricetidae sp. 25 0.2 2.0 0.1 0.8 Microtus agrestis 40 3.0 30.6 4.1 29.2 3.3 30.1 Arvicola terrestris 150 1.3 3.5 0.9 2.3 Rattus norwegicus 225 5.2 9.4 14.7 18.8 7.8 12.8 Sciurus vulgaris 290 4.2 5.9 3.0 3.8 Erinaceus europaeus 765 11.0 5.9 5.5 2.1 9.5 4.5 Ondatra zibethica 1,100 3.2 1.2 2.3 0.8 Lepus timidus 3,900 56.1 5.9 56.5 4.2 56.2 5.3 Small birds < 100 2.4 0.3 1.5 0.2 Mid-sized birds 100–1,000 0.4 6.7 0.3 4.8 Large birds > 1,000 2.4 8.6 4.2 17.1 3.0 11.0 Rana sp. 50 0.7 4.2 0.2 1.5 Number prey 85 48 133 Mean prey/stomach 2.0 2.0 2.0 Mean prey size 828 575 736 Levins’ index 5.996 4.664 food biomass for both sexes. Females took more empty or contained only some unidentified hair, Hedgehogs (Erinaceus europaeus)andmales but some individuals had remarkable numbers of more Brown rats (Rattus norwegicus), both mak- prey items in their stomachs. One male had nine ing more than 10% of their diet biomass. The aver- Harvest mice in its stomach (total stomach weight age weight of the 85 prey items taken by female = 63 g) and another male had three Harvest mice, was 828 g, while the average weight of male prey one Bank vole and five Field voles in its stomach based on 48 items was 575 g. The number of large (total stomach weight = 245 g). One male took two prey above rat size accounted 27.1% for females Brown rats, the average weight of which makes and 10.4% for males, the difference being statisti- 450 g. A stomach from one female contained nine cally significant (¤2 = 4.160, df = 1, p < 0.05). The Field voles, while the highest number of prey difference was much less when taking into account found in a single stomach came from a female that the contribution to the diet biomass: 89.2% and had 14 items: nine Harvest mice, four Field voles 79.1%, respectively.