Diet composition of the Long-eared Owl (Asio otus) in the Eastern Anatolia (Turkey)

In this study, pellet compositions of Long-eared Owl (Asio otus) were analysed. The primary aim of this study was to investigate the diet composition of Long-eared Owl in high altitude steppe in summer. Compositions of 130 pellets of Longeared Owl were used in the study. 147 preys (1.13 preys per pellet) which belong to 9 different taxa were found in pellet composition. A significant part of the diet in study area consisted of small mammals. Only two remains of birds were recorded from pellets. Microtus sp. was found important prey in the diet of Long-eared Owl (F = 81.63 %, B = 78.21%). Since Microtus sp. is the favourite prey of Long-eared Owl, this situation causes a negative correlation with the prey variety in diet composition. For this reason, FNB and Shannon-Wiener indices had low levels, Shannon-Wiener (H’) value was found to be low for Longeared Owl.


Introduction
Diet content studies are helpful in determining the distribution of preys, prey abundance and hunting strategies of birds (Torre et al., 2004). Many different methods such as pellet analysis, stomach content analysis, regurgitation of juvenile birds, fecal content examination of prey remains in the nest and around the feeding area and direct examination are used in finding out prey preferences (Duffy and Jackson, 1986;Marti, 1987). Pellet analysis is a frequently used method in finding out feeding strategies of owls (Terry, 2008). The pellets contain mammal bones such as the skulls and mandibles. Those remains are useful to identification of small mammals at the genus and even species level. Therefore, many significant information can be provided about the composition of small mammal within preying areas of owls and their diet preferences (Yalden, 2009). Long-eared Owl (Asio otus) is an opportunist species which is distributed widely in Holarctic region (Mebs and Scherzinger, 2000). While Vole (Microtus spp.) and Mice (Apodemus spp., Mus spp.) species form the main nutritional source of Long-eared Owl, diet composition can differ depending on geographical and climatic factors (Romanowski and Zmihorski, 2008).
Long-eared Owl is a resident and wintering bird species and also one of the most common owl species throughout most of the country, but rather more local in Eastern Anatolia. It is found in different types of habitat with trees such as forest, wooded areas, plantations areas, parks and gardens in urban areas (Kirwan et al., 2008). The primary aim of this study was to investigate the diet composition of Long-eared Owl in high altitude steppe.

Research area
This study was conducted in the province of Kars in Eastern Anatolia region in 2018 (N40°27'-E42°49'). The pellets were collected in 29 th of July 2018. Pellets of Longeared Owl were obtained from Scots pine (Pinus sylvestris) plantation areas which surrounded by agricultural fields. This plantation area is located within the high steppe at 2200 meter altitude in the province of Kars. The distance of Scots pines to each other is approximate 3 meters. Pellets were scattered in the area and were not collected under a particular tree. The study area is a breeding site of Longeared Owl. The highest observed number is 13 individuals at once in the study area and all of them were juvenile Long-eared Owls. The average temperature in July is 17.6 °C and the average rainfall was 57.6 mm. The average annual temperature was 4.9 °C and the average rainfall is 499.8 mm (MGM, 2018).

Research method and data analysis
Small mammal identification was made according to Vohralik (2005, 2009). Biomass was calculated as the sum of the masses of all individuals of the prey species. The geographic, sex and individual-related differences in terms of biomass can vary among the many species. Other papers containing data on mean values of sex and age groups of prey would be very helpful for the calculations of biomass (Birrer, 2009). For this reason, while calculation of mammal biomass, the data in Vohralik (2005, 2009)'s studies, which reflects Turkey sample, were taken into consideration. Skull and mandibula were used for the identification of small mammals. The minimum number of individuals (MNI) of small mammal and bird taxa inside the pellet composition, frequency (F%), biomass (B%), average prey amount for each pellet and biomass rates were calculated. For statistical analysis, Food Niche Breadth (FNB) index was calculated according to Levin's (1968) formula: 1/Σpi 2 , where pi denotes contribution of a given prey group to the diet. Shannon-Wiener Index (H'; Formula = -∑piln(pi); 'pi' is the proportion of species in the entire sample, 'ln' is the natural logarithm) (Krebs, 1994) which shows the species richness in the diet and the individual numbers between taxa and Evenness Index (J'; formula = H'/logS; S number of the species in the pellets, J'; 0 = individuals not equally distributed, 1 = individuals equally distributed) which shows the distribution of individuals (regular/irregular) within the diet were used.

Results
As a result of the analysis of 130 pellets of Long-eared Owl, 147 preys of nine different small mammal taxa and two bird preys (unidentified) were found in pellet composition (Table 1). Small mammals constitute a significant part of the diet composition (F=98.64%, B=99.28%). Microtus spp. (F=81.63%, B=78.21) is the dominant prey in the diet preference of Long-eared Owl. The remains which belong to Microtus sp. were found in 78% of the pellets. Following Microtus sp., Apodemus sp. (F=6.80%, B=4.84%) was the highest rated taxon within diet composition (Table 1). Although Mesocricetus brandti had a low frequency in diet composition, it was found to be the highest biomass rate, following Microtus sp. It was determined that very low frequency of bird remains in the pellet composition of the Long-eared Owl. While the rate of prey per pellet was 1.13, the rate of biomass was 42.96 g (Table 1).

Discussion and conclusions
In previous studies which have been conducted to find out the diet of Long-eared Owl, most of the diet composition consists of Vole species as dominant preys (Birrer, 2009). Although Vole species (or Sigmodon spp. which are ecologically similar to Vole species, Williford, 2011) constitute the main prey of Long-eared Owl, other prey groups can also have high rates in diet composition (for birds, Kiat et al., 2008;Sándor and Kiss, 2008; for bats, Tian et al., 2015). A total of 475 studies on the dietary content of the Long-eared Owl were collected by Birrer (2009). With the data set obtained in this study, 477 species were identified as prey of the Long-eared Owl. Among these species, 180 (37.74%) were small mammals and 191 (40.04%) were birds. Although the rate of the bird taxa is higher in terms of prey variety, 93.3% of vertebrate prey are small mammal species, 6.4% are birds and 0.3% are other vertebrates. Although the number of bird species is higher than small mammal species, the main preys of Long-eared Owl consist of small mammals. Similarly, small mammal species were dominant preys in this study (98.64%). Göçer (2016) found bird remains at a rate of 100% in an urban area in Southwestern Turkey. Similar of this exceptional situation had been found in a few other studies (Sándor and Kiss, 2008;Kiat et al., 2008). However, pellets in these studies were collected from natural habitats unlike in Turkey. According to the optimal nutritional theory summarized by Pyke (1984), the diversity and rates in the diet depend on the abundance and availability of the most important prey. At the same time, it is stated that the Longeared Owl prey on more birds when small mammals are low density and it is difficult to access to those preys (Milchev and Ivanov, 2016). The reason of that only birds were found from pellets collected in the urban area in Turkey could be that rodent populations are negatively affected by anthropogenic effects. This situation indicates that the Long-eared Owl is an opportunistic predator, as referred in various publications (Bertolino et al., 2001;Tulis et al., 2015).
The most important factors determining the distribution rates of prey in the diet of the Long-eared Owl are probably the size of the prey and its abundance in the area (Birrer, 2009;Pyke, 1984). It is reported that animals lighter than 300 g are potential prey, but animals lighter than 50 g are more dominant, as is the case with Microtus species (Birrer, 2009).
On the other hand, in different habitats, the main prey can consist of Rats Rattus spp. (Pirovano et al., 2000), Wood Mouse Apodemus spp. (Bertolino et al., 2001), and Mice Mus spp. (Song et al., 2010) groups rather than Voles Microtus spp. in diet composition. The reason for this situation could be that Long-eared Owl prefers the more abundant preys in the area because the small mammal composition is related to habitat types.
In studies conducted in different localities and different habitats of Turkey, Voles (Microtus spp.) are the main prey in the diet of Long-eared Owl (Asio otus). The only exception of this situation is the study conducted to find out the diet composition in breeding period in city center populations reported by Göçer (2016) (Table 2). Since Microtus spp. is the favorite prey of Long-eared Owl, this situation causes a negative correlation with the prey variety in diet composition (Mori and Bertolino, 2015). For this reason, FNB and Shannon-Wiener indices have low levels. As recorded in other studies, the diet content of the Longeared Owl depends on different factors which related to each other such as season, geographical location, breeding or non-breeding area, abundance and attractiveness of prey, behaviour of the prey and the climatic conditions (Birrer, 2009). As a conclusion, a significant part of the diet of Long-eared Owls in study area consisted of small mammals. Bird remains were recorded as only two individuals. In order to better understand the diet of Long-eared Owl and to determine which variables affect the diet, more studies are needed with pellets collected from different habitat types, localities and seasons.