Winter Diet of the Barn Owl (Tyto Alba) and the Long-Eared Owl (Asio Otus) in Eastern Poland

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NORTH-WESTERN JOURNAL OF ZOOLOGY 9 (1): 16-22 ©NwjZ, Oradea, Romania, 2013 Article No.: 131601 http://biozoojournals.3x.ro/nwjz/index.html

Winter diet of the barn owl (Tyto alba) and the long-eared owl (Asio otus) in Eastern Poland

Ignacy KITOWSKI

State School of Higher Education in Chelm, Pocztowa 54, PL 22-100 Chelm, Poland. E-mail: [email protected]

Received: 01. September 2011 / Accepted: 08. September 2012 / Available online: 26. December 2012 / Printed: June 2013

Abstract. Prey use is compared between barn owl and long-eared owl from pellets sampled at six sites in eastern Poland. Mammals made up 97.3% of long-eared owl diet by biomass. The most important species were voles Microtus spp. and mice Apodemus spp., constituting 79.9% and 13.0% by biomass, respectively. Their average prey weight was 19.9 g. Mammals made up 99.6% of barn owl diet by number and 99.2% by biomass. The most important were voles Microtus spp. and mice Apodemus spp., constituting 79.9% and 13.0% by biomass, respectively. Average prey weight was 12.3g. A significant difference was found in the mean weight of prey taken by the two owl species. barn owls took less avian prey compared to long-eared owls. Birds caught by long-eared owls were also heavier than those caught by barn owls, though the difference was not significant. The median weight of bird prey for barn owls and long-eared owls was 23 g and 32 g, respectively. The similarity of diet of the wintering owls in particular sites was in the range: 9.0%-98.4%. The total prey overlap of the two wintering species was 83.2%.

Key words: barn owl, long-eared owl, winter diet, sympatry, Poland.

Introduction Material and methods

There have been a few comparative studies on the The study was carried out in 6 locations in the agricultural landscape of the southeastern part of Poland (dis- diets of the barn owl Tyto alba (Scopoli, 1769) and tricts: Tomaszow Lubelski, Hrubieszow), where large- the long-eared owl Asio otus (Linnaeus, 1758). area monocultures of grains, beets and potatoes domi- Winter feed ecology data has been useful in nate. The infrastructure of the farms and their dwellings documenting the trophic relationships in the areas mostly formed “islands” surrounded by large-field where the two species are sympatric in winter monocultures. Within these farms daily roosts of owls (Marks & Marti 1984, Capizzi & Luiselli 1996, were located. Pellets of Long-eared owls were collected at Alivizatos & Goutner 1999, Petrovici et al. 2013). communal winter roosts of 9 to 29 birds, situated on co- niferous trees within the area of farms. Barn owl pellets Central Europe, including Poland, lies within the were collected in neighboring granaries, barns, cowsheds, wintering and breeding areas of the considered corn and hop bins, sheds, fertilizer storehouses, etc. They species of owls (Mikkola, 1983). However, data is originated from 1-3 wintering roosting individuals. missing from this part of Europe on the food ecol- The collection of pellets of both species was preceded ogy of owls in cases when they are sympatric in by the removal of old pellets, cast previously in the pellet winter. Data on the winter diet of the long-eared stations, on the last day of astronomical autumn 2005 in 6 owl mostly originates from the central and west- sites of their coexistence. On average, the pellet stations of both species were situated 196 m from one another (70- ern parts of Poland (Cichocki et al. 2008, Ro- 323 m). Winter pellets of both species were collected on manowski & Zmihorski 2008, Kitowski et al. 2005). the first day of astronomical spring of 2006 in the follow- On the other hand, a substantial amount of data ing villages: Wierzbica, Wolka Wierzbicka, Kornie, and on the diet of the barn owl has come mainly from Krzewica in Tomaszów Lubelski district as well as Ma- the collections of mammalogists, where pellets jdan and Kosciaszyn in Hrubieszów district (Fig. 1). The from different parts of the year are mixed (Ru- pellets were prepared for analysis by standard methods precht et al. 1998). Studies containing data on the (Ruprecht et al. 1998). Following several authors (Arnold & Burton 1980, Pucek 1984, Cuisin 1989), the number of winter diet of barn owls from Central Europe are vertebrate prey species was determined on the basis of extremely scarce (Baranowski & Tryjanowski skulls, mandibles, teeth, and other important remains 1999). This study compares the diet of sympatri- such: pelvis bones, beaks and claws. Some prey was cally wintering barn owls and long-eared owls in 6 grouped, for example: Sylvaemus, Muridae, etc. (see Table locations in a farmland area of southeastern Po- 1), because sometimes a high degree of fragmentation did land. not allow identification of the prey. Data for mammalian and avian prey biomass were taken from: Pucek (1984), Romanowski (1988) and Jedrzejewska & Jedrzejewski Winter diet of Tyto alba and Asio otus in Eastern Poland 17

(2001). Totally 1114 and 687 pellets of long-eared owls mice Apodemus spp. (11.5% by number and 13.0% and barn owl were analyzed, respectively. by biomass). Other mammalian prey (including Insectivora such as shrews Sorex sp. and synanthropic mammals such as house mouse Mus musculus L, 1758 or norway rat. Rattus norvegicus (Berkenhout, 1769), harvest mouse Micromys minutus (Pallas, 1771), common dormouse Muscardinus avellanarius L, 1758 and as big biomass prey as weasel Mustela nivalis L, 1766 – were diet constitu- ents of lower importance. Avian prey (at least 7 species) formed 1.5% of the diet by number and 2.7% by biomass. Average prey weight was 19.9 ± 8.4 g (median: 19 g., range: 3.5 g – 310 g). Prey diversity for all studied sites was H = 1.734, and the width of food niche was B = 1.948. The width of the Levin’s food niche indi-

ces of long-eared owl was the highest at Wierzbica Figure 1. Location of pellet collection sites site (B = 5.87) and the smallest at Krzewica site (B (1 – Wierzbica, 2 – Wolka Wierzbicka, 3 – Kornie, 4 – Krzewica, 5 – Majdan, = 1.48). Data for particular other pellet stations are 6 – Kosciaszyn). given in Table 2. Mammals made up 99.6% of barn owl diet by number and 99.2% by biomass. At least 16 mam- The width of the food niches of owls was estimated mal prey species were found. The most important using the formula by Levins (1968): B = 1/∑ pi2, where pi is the proportion of the prey category i in the total bio- of them were voles Microtus spp. (31.4% by num- mass of the owl’s diet. Shannon-Wiener diversity (H) in- ber and 50.1% by biomass), shrews Sorex spp. dices were calculated for trophic diversity at species lev- (55.7% by number and 31.0% by biomass), mice els. Using the formula, H = - ∑ [Pi log (Pi)] (Krebs 1994). Apodemus spp. (5.6% by number and 8.2 % by The Renyi –function (Tóthmérész 1995) was used to biomass). Birds were unimportant as prey. Aver- characterize prey diversity of pellet stations. The Renyi – age prey weight was 12.3 ± 13.6 g (median: 8 g, diversity: H α = (log ∑ (Ni/NT) α )/(1- α) where 0 < α,< α range: 3.5 g - 310 g). The width of the Levin’s food ≠1,Ni = means the number of individuals of number i, T niche indices of barn owl was the highest at Kosci- means the total number species. NT = means thetotal number of individuals and α is a scale parameter. Where aszyn site (B = 6.13) and the smallest at Majdan the scale parameter is low the function is very sensitive to site (B = 1.50). Overll prey diversity for barn owl rare prey species, whereas high values of the scale pa- was H = 2.44 and the width of food niche was B = rameter suggests that function is more sensitive to domi- 3.89, both being higher than those of the long- nant species. If α → 1, then H α → Hs (Shannon diversity). eared owl. Data for particular pellet stations this Trophic niche overlap was measured through Pi- species are given in Table 2. anka’s index, using the percentage of biomass consumed Low weight prey (mainly insectivores weigh- of particular food items (Ojk = ∑ pij pik /√∑ pij2 ∑ pik2, where pi is the percentage of prey item “i” in the diet of ing up to 8 g) were more important for barn owl species “j” and “k”) (Pianka 1973). Pianka’s index varies (60.2% vs. 2.0%). A highly significant difference between 0 (total separation) and 1 (total overlap). It was was found in the median weight of prey taken by multiplied by 100 and expressed as a percentage. The the two owl species (Mann-Whitney Z = - 29.36, n1 means and standard deviation (SD) are provided for pa- = 1236, n2 = 1893, p < 0.00001). rametric data while non-parametric data are presented as Barn owls caught less avian prey compared to medians with ranges (Fowler & Cohen 1992). long-eared owls (Table 1). Birds caught by long- eared owls were heavier than those caught by Results barn owls median 32g (range: 14.5-110g) vs. median: 23g, (range: 23-32g), respectively. However, Mammals made up 98.5% of the diet of long-eared the differences were not significant (Mann- owls by number and 97.3% by biomass. At least 17 Whitney Z = -1.75, n1 = 5, n2 = 28, n.s.). mammal species were identified in the diet. The The total prey overlap of the two wintering most important of them were voles Microtus spp. species, expressed through Pianka’s (1973) index (81.8% by number and 79.9% by biomass) and was 0.832 (83.2%), and mammalian prey overlap

18 I. Kitowski

Table 1. Prey species of Barn Owl Tyto alba and Long-eared Owl Asio otus during winter in East Poland.

Mass Barn Owl Long - eared Owl Species [g] N %M %N %M Sorex araneus L, 1758 8.0 41.50 26.98 0.05 0.02 Sorex minutus L, 1758 3.5 14.16 4.03 0.05 0.01 Neomys fodiens (Pendant, 1771) 14.0 0.24 0.28 0.05 0.04 Crocidura leucodon (Hermann, 1780) 8.0 1.13 0.74 0.20 0.09 Myodes glareolus (Schreber. 1780) 17.0 0.16 0.22 0.70 0.63 Microtus subteraneus (de Selys-Longchamps, 1836) 17.0 0.65 0.89 1.70 1.49 Microtus oeconomus (Pallas, 1776) 26.0 1.94 4.10 4.10 5.40 Microtus agrestis (Pallas, 1761) 23.0 0.08 0.15 0.50 0.55 Microtus arvalis (Pallas, 1779) 19.0 26.94 41.59 74.10 70.89 Microtus sp. 23.0 1.80 3.33 1.40 1.59 Mus musculus L, 1758 15.5 1.30 1.63 2.10 1.65 Rattus norvegicus (Berkenhout, 1769) 310.0 0.16 4.08 0.05 0.82 Micromys minutus (Pallas, 1771) 8.0 3.40 2.21 1.70 0.70 Apodemus agrarius (Pallas, 1771) 17.0 4.69 6.48 5.70 4.89 Apodemus flavicollis (Melchior, 1834) 31.0 0.24 0.61 4.10 6.43 Apodemus silvaticus L, 1758 20.0 0.08 0.13 1.20 1.17 Apodemus sp. 22.5 - - 0.40 0.42 Sylvaemus 20.5 0.57 0.94 0.10 0.11 Muridae 18.0 0.40 0.59 0.10 0.10 Muscardinus avellanarius L, 1758 16.0 0.16 0.21 0.05 0.04 Mustela nivalis L, 1766 80.0 - - 0.05 0.21 Turdus pilaris L, 1758 110.0 - - 0.10 0.58 Parus major L, 1758 18.0 - - 0.10 0.10 Parus sp. 14.5 - - 0.05 0.04 Passer domesticus L, 1758 32.0 0.08 0.21 0.90 1.36 Passer montanus L, 1758 23.0 0.32 0.60 0.30 0.31 Coccothraustes coccothraustes L, 1758 55.0 - - 0.05 0.15 Emberiza citrinella L, 1758 30.0 - - 0.05 0.08 Emberiza calandra L, 1758 50.0 - - 0.05 0.13 Total prey number - 1236 1893 Total prey biomass [g] - 15213.0 37578.5

Table 2. Indices describing food habits of Barn owls and Long-eared owls. BTA – the width of the Levin’s food niche indices of Barn owl Tyto alba. %M.arv TA – percentage of Common vole Microtus arvalis in the consumed biomass of Barn Owl. BAO – the width of the Levin’s food niche indices of Long-eared owl Asio otus, % M.arv AO – percentage of Common vole Microtus arvalis in the consumed biomass of Long-eared owl. HTA – Shannon-Wiener diversity indices of Barn owl. HAO – Shannon-Wiener diversity indices of Long-eared owl. OI – Pianka’s indices of trophic niche overlap for both owl species.

% M. % M Sites B TA B AO H TA H AO O I arv TA arv AO Majdan 1.50 0.81 1.80 0.72 1.41 1.60 0.800 Wierzbica 2.75 0.04 5.87 0.29 1.18 2.78 0.090 Wolka Wierzbicka 3.00 0.50 1.88 0.71 2.22 1.63 0.868 Kosciaszyn 6.13 0.22 1.49 0.80 2.74 1.15 0.566 Kornie 1.97 0.13 1.57 0.79 1.44 1.35 0.188 Krzewica 2.20 0. 65 1.48 0.77 2.10 1.44 0.984 Overall 3.89 - 1.95 - 2.44 1.73 0.832

was 0.833 (83.3%). The similarity of diet of the biomass of barn owls was associated with an in- wintering owls was the highest at Krzewica site O significant (Spearman r = -0. 37, n = 6, p = 0.46) = 98.4%, and the smallest one was at Wierzbica decrease in width of the Levin’s food niche indices (only 9.0%) (Table 2). An increase in the rate of and a significant increase of Pianka's overlap in- common vole Microtus arvalis in consumed prey dex (Spearman r = 0. 83, n = 6, p = 0.04). An in- Winter diet of Tyto alba and Asio otus in Eastern Poland 19 crease in the rate of common vole in eaten prey For both owl species, an increase in the H di- biomass of long owls lead to a significant de- versity index was not associated with a respective crease in width of the Levin’s food niche indices increase in Pianka's overlap index: Spearman r = (Spearman r = -0.83, n = 6, p = 0.04) and an insig- 0.486, n = 6, p = 0.329 and Spearman r = -0.086, n = nificant increase of Pianka's overlap index 6, p = 0.871 for barn owl and long-eared owls, re- (Spearman r = 0. 06, n = 6, p = 0. 87). An increase spectively. Analysis of Renyi diversty profiles (Fig in the rate of common vole Microtus arvalis in 2, Fig 3) showed small diversity of prey for barn consumed prey biomass led to significant lower- owls with clear separateness of Kornie pellet sta- ing of H diversity index for long – eared owl tion. For both barn owl and long-eared owls was (Spearman r = -0.99, n = 6, p = 0.001) but not for typical high similarity at Krzewica and Majdan barn owl (Spearman r = 0.12, n = 6, p = 0. 83) pellet stations.

Figure 2. Prey diversity profile for the Barn owl Tyto alba.

Figure 3. Prey diversity profile for Long –eared owl Asio otus.

20 I. Kitowski

Discussion owls from western Poland switched their preda- tion pressure during winter to alternative prey – Small mammals were the most important prey of the house mouse and the house sparrow Passer both barn owls and long-eared owls wintering in domesticus L, 1758. Similar tendencies shaped the southeastern Poland. In that region, barn owls had diet composition of the studied owl in eastern Po- a broader diet than long-eared owls. In many land. As in Baranowski and Tryjanowski's (1999) other areas where barn and long-eared owls coex- studies, the only birds found in the winter diet of isted, including wintering areas, barn owls had barn owls were sparrows (Passer sp.). Yet, unlike also been shown to have a broader diet (Marti in Baranowski and Tryjanowski's (1999) results, 1976, Amat & Soriguer 1981, Veiga 1981, Capizzi where the average proportion of birds in total & Luiselli 1996, Alivizatos & Goutner 1999). This biomass during winter months was about 25%, in probably resulted from the high dietary overlap eastern Poland sparrows played a marginal role in between the two species and may have facilitated winter diet (0.81 % of total biomass). Baranowski their coexistence in areas of syntopy (Marks & and Tryjanowski (1999) found that the proportion Marti 1984). of common shrews in prey biomass during winter The composition of the food of the studied months did not exceed 2%, and in January and owls showed that there was a spatial separation of February they did not report any common shrews. habitats used for hunting. Barn owls preferred Studies (Bose & Guidali 2001, Cichocki et al. 2008), foraging in damper areas (meadows, mid-field report that the winter decrease in the availability depressions, and the surroundings of buildings of voles for barn owls led to an increase in the (Bunn et al. 1982, Capizzi & Luiselli 1996, Aliviza- proportion of shrews in barn owl diet. This sug- tos & Goutner 1999, ), avoiding woodlots. Long- gests that the barn owls studied in southeastern eared owls hunted in fields not avoiding woodlots Poland may switch prey use in time; they exhibit (Paunović et al. 2005). These differences were most an opportunistic feature in their ability to switch clearly reflected in the low importance of birds in between prey items, the way selective opportunist the winter diet of barn owls. A very different predators do (Colvin & McLean 1986, Tores et al. situation was observed in Greece, where in one of 2005). While some authors conclude that the barn the locations birds constituted 15% of the diets of owl is a selective predator selecting to prey on cer- both wintering species by biomass (Alivizatos & tain species (Bunn et al. 1982), others maintain that Goutner 1999). The very clear differences in bird this owl is an opportunist predator (Charter et al. species composition in the diets of the two owl 2007, Bernard et al. 2010). species in eastern Poland may have been related to According to Zmihorski (2005), the breadth of differences in their hunting behavior (Alivizatos & winter niche of the long-eared owl, measured us- Goutner 1999). Although both forage in open ar- ing B Levins index, was 1.1 for an farmland loca- eas, long-eared owls can hunt under tree canopy tion and 4.0 for a forest location. In central Poland as well, which may account for the higher propor- Cichocki et al. (2008) found the breadth of winter tion of Apodemus species taken (8.2% vs. 11.2%), trophic niches of the long-eared and barn owls as: and they also raid bird roosts in bushes and trees B =1.9 and B = 2.4, respectively. to a much greater extent than barn owls. This These results correspond with present data of brought effects in the form of greater number and SE Poland (range: 1.48-5.87), and of those reported biomass proportion of birds in their diet – respec- in an earlier study on the diet of the long-eared tively 6 and 8 times greater than in barn owl diet. owl wintering in the agricultural landscape of Sometimes local high specialization on avian prey eastern Poland, where B Levins index ranged be- may occur on long eared owls (Sándor & Kiss tween 1.15 and 5.7 (Kitowski et al. 2005), pointing 2008). However, the presence of synanthropic to a narrowing of food niche breath. mammals shows that both species may have also Other studies also point to a narrowing of the hunted near buildings. Barn owls have long been food niche in winter (Nilsson 1981, Wijnanddts known to do so (Bunn et al. 1982). In the case of 1984, Galeotti & Canova 1994). Only Tome (1994) long-eared owls, this behaviour probably occurs for long-eared owl demonstrated that at locations sporadically, as has already been indicated in ear- in Central Slovenia a noticeable broadening of the lier studies (Kitowski et al. 2005). food niche occurred in winter. Dietary overlap re- According to Baranowski and Tryjanowski flected by Pianka’s index of the two wintering (1999), when the density of vole was low, barn species varied substantially among the areas stud- Winter diet of Tyto alba and Asio otus in Eastern Poland 21 ied, averaging 83.2% for all the six sites, a result Arnold, E.N., Burton, J.A. (1980): A Field Guide to the Reptiles and Amphibians of Britain and Europe. Collins, London. similar to Greece being 86% (Alivizatos & Goutner Baranowski, P., Tryjanowski, P. (1999): [Seasonal variation in the diet 1999). Spanish data from winter (Delibes et al. of barn owl Tyto alba (Scop., 1769) near Konin, central Poland] 1983) showed that dietary overlap between long- Rocznik Naukowy Polskiego Towarzystwa Ochrony Przyrody “Salamandra” 3: 99-106. [in Polish] eared owls and barn owls was 89%. Two-year Bertolino, S., Ghiberti, E., Perrone, A. (2001): Feeding ecology of the breeding data from Idaho, USA, showed that die- long-eared owl (Asio otus) in northern Italy: is it a dietary tary overlap between barn and long-eared owls specialist? Canadian Journal of Zoology 79: 2192-2198. Bernard, N., Michelat, D., Raoul, F., Quéré, J.P., Delattre, P., was 48% the first year and 61% the next year Giraudoux, P. (2010): Dietary response of Barn Owls (Tyto alba) (Marks & Marti 1984). Year round studies from to large variations in populations of common voles (Microtus western Romania showed that dietary overlap be- arvalis) and European water voles (Arvicola terrestris). 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