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Spatial Variation of Eurasian Eagle-Owl Diets in Wetland andNon-Wetland Habitats in West-Central KoreaAuthor(s): Dong-Man Shin and Jeong-Chil Yoo Da-Mi JeongSource: Journal of Raptor Research, 47(4):400-409. 2013.Published By: The Raptor Research FoundationDOI: http://dx.doi.org/10.3356/JRR-13-00006.1URL: http://www.bioone.org/doi/full/10.3356/JRR-13-00006.1

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SPATIAL VARIATION OF EURASIAN EAGLE-OWL DIETS INWETLAND AND NON-WETLAND HABITATS IN

WEST-CENTRAL KOREA

DONG-MAN SHIN AND JEONG-CHIL YOO1

Korea Institute of Ornithology and Department of Biology, Kyung Hee University, Seoul 130-701, Korea

DA-MI JEONGDivision of Molecular and Life Sciences, Ewha Woman’s University, Seoul 120-750, Korea

ABSTRACT.—The diet of the Eurasian Eagle-Owl (Bubo bubo) was studied in two different land-cover types:wetlands and non-wetlands. We analyzed 1458 prey items obtained from 924 pellets and 534 prey remainscollected in 34 territories in west-central Korea. Diet composition differed significantly between the twohabitats: diet was mainly birds (68.9% by number; 85.3% by biomass) in wetlands, but was dominated bymammals (38.7% by number; 64.7% by biomass) in non-wetlands. At the species level, ducks such asEastern Spot-billed Ducks (Anas zonorhyncha) and Mallards (Anas platyrhynchos) made up 38.1% by biomassin wetlands, whereas brown rats (Rattus norvegicus) were the most abundant prey by number and biomass innon-wetlands. The mean weight of prey: (a) was greater in wetlands than in non-wetlands; (b) was positivelyrelated to the density of wetlands in the study area and (c) negatively related to both the percentage ofagricultural land and the density of human settlements. The diet of Eurasian Eagle-Owls apparently reflectsthe differential abundance of local food resources. The values of diet diversity (H9) and standardized food-niche breadth (Bst) were relatively high in wetland habitats.

KEY WORDS: Eurasian Eagle-Owl; Bubo bubo; biomass; diet; pellet analyses; wetlands.

VARIACION ESPACIAL DE LAS DIETAS DE INDIVIDUOS DE BUBO BUBO EN HABITAT DE HUME-DALES Y NO HUMEDALES EN EL OESTE CENTRAL DE COREA

RESUMEN.—Se estudio la dieta de Bubo bubo en dos tipos diferentes de cobertura de tierras: humedales y nohumedales. Analizamos 1458 ıtems de presa obtenidos de 924 egagropilas y 534 restos de presas colectadosen 34 territorios en el centro oeste de Corea. La composicion de la dieta difirio significativamente entre losdos habitats: la dieta estuvo compuesta principalmente por aves (68.9% por numero; 85.3% por biomasa)en los humedales, pero estuvo dominada por mamıferos (38.7% por numero; 64.7% por biomasa) en losno humedales. A nivel de especie, los patos como Anas zonorhyncha y A. platyrhynchos comprendieron 38.1%de la biomasa en los humedales, mientras que Rattus norvegicus fue la presa mas abundante por numero ybiomasa en los no humedales. El peso promedio de presa: (a) fue mayor en los humedales que en los nohumedales; (b) estuvo positivamente relacionado con la densidad de humedales en el area de estudio y (c)negativamente relacionado tanto con el porcentaje de tierra agrıcola como con la densidad de asentamien-tos humanos. La dieta de B. bubo aparentemente refleja la abundancia diferencial de fuentes de alimentolocal. Los valores de la diversidad de la dieta (H9) y el ancho estandarizado del nicho alimenticio (Bst)fueron relativamente elevados en los habitats de los humedales.

[Traduccion del equipo editorial]

The Eurasian Eagle-Owl (Bubo bubo), widely dis-tributed across Europe, Asia, and North Africa (Mik-kola 1994, Penteriani 1996) has been reported invarious habitat types, such as forests, shrublands,openlands, croplands, pastures, and human-alteredareas (Jaksic and Marti 1981, Martınez and Zubero-

goitia 2001, Marchesi et al. 2002, Penteriani et al.2004). It is a nocturnal top-predator with opportu-nistic foraging behavior (Jaksic and Marti 1984).Although it feeds primarily on mammals (Koniget al. 1999, Martınez and Zuberogoitia 2001, Marchesiet al. 2002, Penteriani et al. 2002), it also eats a varietyof birds, reptiles, amphibians, insects, and even car-casses (Konig et al. 1999). However, mammals are the1 Email address: jcyoo@khu.ac.kr

J. Raptor Res. 47(4):400–409

E 2013 The Raptor Research Foundation, Inc.

400

primary prey consumed by the eagle-owl in mosthabitats. Although it takes a wide range of preyspecies, its diet in any particular location is usuallydominated by one or two species, typically thosethat are locally abundant and profitable (Marchesiet al. 2002).

Over the last five decades, large areas of mud flatswere reclaimed in western Korea for economicdevelopment. The Sihwa Reclaimed Area (SRA),which was constructed in 1994 (K-water 2009), is atypical reclaimed area in Korea. This area hascaused a number of environment problems, suchas water pollution, fish kills, and destruction of hab-itats important to migratory shorebirds. However,the reclaimed area supplied an unexpectedly attrac-tive habitat for the Eurasian Eagle-Owl. The rockycliffs located near the dried-up shore were used fornesting sites. The wide range of artificial wetlands,reed beds, and fallow lands that were created afterthe reclamation attracted large numbers of birds,including Ring-necked Pheasants (Phasianus colchi-cus) and waterfowl, which were suitable prey foreagle-owls.

The land-cover types surrounding a nest site andthe heterogeneous distribution of resources withinthe landscape affect the distribution, density, diet,and reproductive rate of eagle-owls (Penteriani et al.2004), which are usually considered a site-depen-dent species (Rodenhouse et al. 1997). EurasianEagle-Owls often nest near a favored huntingground (Frey 1973, Donazar 1988, Leditznig 1992,Penteriani et al. 2004); thus, in wetlands, wheremammals are relatively scarce and waterbirds areabundant, eagle-owl diet would be expected to in-clude many birds.

Thus, we hypothesized that large-sized birdswould be the main prey of eagle-owls in wetlandsand would positively affect its mean weight of prey(MWP). In this study, we investigated eagle-owl dietin both SRA wetlands and non-wetlands, i.e., agri-cultural lands, forests, and human settlements. Ourgoal was to assess whether the diet composition,MWP, and trophic diversity indices differed signifi-cantly between the two different habitat types.

METHODS

Study Areas. The two study areas were located inthe west-central region of Korea (Fig. 1): (1) theSRA (ca. 360 km2) located near Ansan City andHwasung City in the Gyeonggi Province (37u49–37u179N, 126u349–126u509E), and (2) the coastalregion from Paju City and Gimpo City in the

Gyeonggi Province to Ganghwa District in IncheonMetropolitan City (PGG; ca. 810 km2; 37u369–37u499N, 126u219–126u519E). The SRA was approxi-mately 50 km south of the PGG.

The SRA landscape was characterized by a gentlemountain slope, with elevations below 150 m asl(range 5 28–146 m) and covered primarily by de-ciduous forests and mixed deciduous-coniferousforests. Artificial wetlands and open lands were wide-ly distributed throughout the study area (Table 1),where halophytes such as Suaeda japonica, Suaeda mar-itima, Salicornia herbacea, and Suaeda asparagoides hadpreviously dominated, but where Calamagrostis epige-jos, Imperata cylindrica var. koenigii, and reeds (Phrag-mites australis) have recently become dominant, as aresult of desalination (K-water 2009). A substantialportion of the wetlands has been converted to ricepaddy fields and a factory complex. For the PGG,deciduous forests and mixed deciduous-coniferousforests were also dominant. However, different land-scapes were encompassed: Ganghwa was dominatedby hills (range 5 42–468 m), whereas Paju andGimpo were intensively cultivated and urbanized,and thus nest sites were largely located close to thetowns (Table 1).

Nest-searching. We studied diet at 34 eagle-owlterritories (n 5 11 for SRA, n 5 23 for PGG) duringthe period from September 2010 to August 2011.Prior to the diet study, we found eagle-owl nestsby various means, including: (i) listening for spon-taneous territorial vocalizations; (ii) eliciting territo-rial calls by using playback of conspecific vocaliza-tions; (iii) observing perched or flying individuals atdusk near potentially suitable cliffs; (iv) visiting ar-eas around historical nest or perch sites to look forrecently molted feathers, fresh pellets, and prey re-mains; and (v) visiting areas where injured eagle-owls had been rescued by community rangers.

Land-cover Analyses. The landscape data for GIS(Geographic Information System) analysis were ob-tained from the classified Arirang two satellite im-ages with a 1-m spatial resolution. These data wereclassified in 23 land-cover categories at pixel level(40 3 40 m) by the Ministry of Environment (ME)of Korea (2009). The landscape data of these digitalmaps were reclassified in seven larger land-covercategories, following the classification scheme ofME (2002): agricultural lands, forests, grasslands,wetlands, human settlements (including houses,buildings, industry, business, resorts, traffic, andpublic areas), fallow lands, and water bodies. Afterplotting a circle with a 1-km radius centered on each

DECEMBER 2013 SPATIAL VARIATION OF EAGLE-OWL DIET 401

nest (e.g., Frey 1973, Donazar 1988, Leditznig 1992,Penteriani et al. 2004), we analyzed the percentagesof each land-cover type using ArcGIS 9.3 (ESRI2008).

Diet Analyses. We collected pellets and prey re-mains at nests, feeding sites, or traditional roostingplaces once or twice a month throughout the yearduring the study period, so as to avoid a bias asso-ciated with possible temporal variation in diet (Oroand Tella 1995, Marchesi et al. 2002, Penteriani et al.

2002). Skulls, mandibles, bones, and feathers wereused for species identification of prey items by com-parison to a reference collection (Lee et al. 2002,Yoon et al. 2004, Takada and Kanochi 2004, Abe2007). We avoided duplicating the count of any preyitem by conservatively choosing the lowest numberof items per collection event (Marti 1987, Murphy1997, Bose and Guidali 2001, Marchesi et al. 2002).

Biomass data for each prey species were derivedfrom the average of body masses in Won (1981),

Figure 1. Study areas in west-central Korea, where information on diet of Eurasian Eagle-Owls was collected duringSeptember 2010 to August 2011.

402 SHIN ET AL. VOL. 47, NO. 4

Yoon et al. (2004), Song and Song (2005), andHume (2006), and our own measurements. Eachinsect was assigned a weight of 1.0 g (Seamansand Gutierrez 1999). Biomass was calculated by mul-tiplying the average body mass by the number ofindividuals in the diet (Jaksic and Marti 1981, Do-nazar et al. 1989, Bose and Guidali 2001, Forsmanet al. 2001). The percentages of frequency and bio-mass were estimated for each prey species based onthe total number and biomass.

Comparison of differences in eagle-owl diets be-tween the two study areas was made in three ways,following Jaksic and Marti (1984): (1) taxonomiccomposition of the diet, (2) trophic diversity, and(3) MWP in the diet of each territory. To assessdifferences in diet composition, we compared theoccurrence frequency of the important prey by x2

analysis on the contingency tables. To avoid cellswith inadequate expected frequencies, prey itemsthat made up .3% of the diet by number or by mass(Ardeidae, Anatidae, Phasianidae, Rallidae, Colum-bidae, Corvidae, other birds, Muridae, Leporidae,Mustelidae, other mammals, insects and crusta-ceans) were grouped in the ‘‘main prey categories’’(Marchesi et al. 2002). The diversity of diet compo-sition was calculated on the frequency of prey spe-cies detected in the diet of each territory by threemethods: the Shannon-Wiener diversity function(H9), a food-niche breadth (B) calculated throughLevins’ (1968) formula, and a standardized food-niche breadth (Bst) proposed by Colwell and Futuyma

(1971). Except for unidentified Calosoma spp., Ember-iza spp., and Rattus spp., unidentified prey items werenot included in calculations of these diet diversityindices.

Statistical Analyses. A stepwise multiple linearregression was used to assess the relationships be-tween land-cover categories and MWP, with entryand removal criteria set at probability of F # 0.05and $0.10 respectively. In this case, the percent-ages of land-cover categories were arcsine-trans-formed, and territories with fewer than 20 preyidentified were excluded from the regression.The Mann-Whitney U-test was used to test the sig-nificance of differences for land-cover categories,MWP, and food-niche breadth (B) between two re-gions, and Student’s t-test was used for dietary diver-sity (H9) and standardized food-niche breadth (Bst).Statistical significance was set at a , 0.05. Statisticalanalyses were performed using PASW Statistics 18.0for Windows (SPSS Korea Datasolution Inc. 2009).All data shown are mean 6 standard error (SE).

RESULTS

The SRA was dominated by wetlands, which rep-resented about half the land cover within the studyarea (Table 1). For the PGG, agricultural land andforest made up .65%. The percentages of agricul-tural land, forest, wetland, and human settlementdiffered significantly between the two study areas(Mann-Whitney U-test), but the percentages of fal-low land, water bodies, and grasslands did not.

Table 1. Land-cover composition of wetlands (SRA) and non-wetlands (PGG) based on 34 eagle-owl territories in west-central Korea, analyzed using GIS data within a 1-km radius centered on each nest. The z-values and P-values (Mann-Whitney U-test) indicate the statistics and significances of the difference of each land-cover category between the twostudy areas, respectively (arcsine-transformed). The number of sample sites 5 n. Figures are represented as meanpercent 6SE.

LAND-COVER CATEGORY

SRAa PGGb

z P

% %

(n 5 11) (n 5 23)

Fallow landNS 8.3 6 4.15 3.9 6 0.97 20.460 0.645Agricultural land 20.4 6 4.38 34.7 6 3.95 22.154 0.031Forest 14.7 6 3.14 30.7 6 4.50 22.264 0.024Water bodyNS 7.3 6 2.64 8.6 6 2.83 20.055 0.956Wetland 44.7 6 5.19 1.7 6 0.59 24.754 ,0.001GrasslandNS 1.2 6 0.59 2.9 6 0.81 21.786 0.074Human settlement 3.5 6 0.59 17.6 6 2.06 24.031 ,0.001

a Sihwa Reclaimed Area.b Paju City, Gimpo City, and Ganghwa District.NS not significant.

DECEMBER 2013 SPATIAL VARIATION OF EAGLE-OWL DIET 403

Table 2. Diet of Eurasian Eagle-Owls in a wetland-dominated landscape (SRA) and non-wetlands (PGG) in west-centralKorea during September 2010 to August 2011 (n 5 34 owl territories). Number of each species (n), frequency of eachspecies (Freq) and biomass of prey (Mass) in the diet are shown. Taxonomic groups accounting for ,1% of number orbiomass of prey items were included in the categories ‘Other mammals’ and ‘Other birds’. tr 5 trace (,0.01%).

PREY SPECIES OR GROUP

SRAa PGGb POOLED

FREQ MASS FREQ MASS FREQ MASS

n % % n % % n % %

Birds (Total) 255 68.9 85.3 459 38.7 64.7 714 45.8 72.6Egretta spp.c 13 3.5 3.6 15 1.3 2.4 28 1.8 1.8Nycticorax nycticorax 3 0.8 1.1 4 0.3 0.9 7 0.4 1.0Ardea cinerea - - - 2 0.2 0.8 2 0.1 0.5Anas acuta 3 0.8 1.6 - - - 3 0.2 0.6A. crecca 7 1.9 1.5 5 0.4 0.6 12 0.8 0.9A. zonorhyncha 32 8.6 19.7 24 2.0 8.8 56 3.6 12.8A. platyrhynchos 18 4.8 11.3 10 0.8 3.7 28 1.8 6.6Other ducksd 9 2.4 4.0 8 0.7 1.9 17 1.1 2.7Anser spp.e 3 0.8 4.6 2 0.2 1.6 5 0.3 2.7Rallidaef 16 4.3 3.1 1 0.1 0.1 17 1.1 1.2Phasianus colchicus 37 9.9 19.4 47 4.0 14.6 84 5.4 16.4Domestic chickeng - - - 4 0.3 2.7 4 0.3 1.7Streptopelia orientalis 71 19.0 10.2 258 21.8 21.9 329 21.1 17.5Pica pica 14 3.7 1.9 32 2.7 2.6 46 3.0 2.3Strigidaeh 2 0.5 0.2 12 1.0 0.9 14 0.9 0.7Other raptorsi 5 1.3 0.8 3 0.3 0.2 8 0.5 0.4Hypsipetes amaurotis 6 1.6 0.2 5 0.4 0.1 11 0.7 0.2Turdidaej 4 1.1 0.2 9 0.8 0.4 13 0.8 0.3Other birdsk 12 3.2 1.9 18 1.5 0.4 30 1.9 1.0

Mammals (Total) 117 31.3 14.7 718 60.6 35.3 835 53.6 27.6Rattus spp.l 44 11.8 5.1 395 33.3 26.9 439 28.2 18.8Apodemus agrarius 27 7.2 0.5 202 17.0 2.1 229 14.7 1.5Mus musculus 2 0.5 tr 41 3.5 0.2 43 2.8 0.1Clethrionomys spp. 23 6.1 0.5 61 5.1 0.7 84 5.4 0.6Lepus coreanus 4 1.1 5.4 3 0.3 2.4 7 0.4 3.5Crocidura lasiura 8 2.1 0.1 3 0.3 tr 11 0.7 trMustela sibirica 6 1.6 2.5 8 0.7 2.0 14 0.9 2.2Other mammalsm 3 0.8 0.7 5 0.4 1.0 8 0.5 0.9

Insects (Total)n 1 0.3 tr 7 0.6 tr 8 0.5 trCrustaceans (Total)u 1 0.3 tr 1 0.1 tr 2 0.1 trTotal prey items 374 1185 1559

a Sihwa Reclaimed Area; b Paju City, Gimpo City, and Ganghwa District; c Little Egret (Egretta garzetta; n 5 12), unidentified egrets (10),Intermediate Egret (Mesophoyx intermedia; 4), Great Egret (Casmerodius albus; 2); d unidentified ducks (Anas spp.; 12), Mandarin Duck (Aix

galericulata; 2), Domestic duck chick (Anas platyrhynchos var. domesticus; 1), Eurasian Wigeon (Anas penelope; 1), Tufted Duck (Aythya

fuligula; 1); e Greater White-fronted Goose (Anser albifrons; n 5 4), Bean Goose (Anser fabalis; 1); f Common Moorhen (Gallinula chloropus;n 5 12), Common Coot (Fulica atra; 3), Water Rail (Rallus aquaticus; 1), White-breasted Waterhen (Amaurornis phoenicurus; 1). g Gallus

gallus domesticus; h Long-eared Owl (Asio otus; n 5 8), Brown Hawk-Owl (Ninox scutulata; 2), Collared Scops-Owl (Otus bakkamoena; 2),Short-eared Owl (Asio flammeus; 1); i Eurasian Kestrel (Falco tinnunculus; n 5 6), Eurasian Sparrowhawk (Accipiter nisus; 2), NorthernHarrier (Circus cyaneus; 1); j Dusky Thrush (Turdus naumanni; n 5 9), Eurasian Scaly Thrush (Zoothera dauma; 2), Pale Thrush (Turdus

pallidus; 2); k unidentified passeriformes (n 5 14), Common Snipe (Gallinago gallinago; 2), Asian Dollarbird (Eurystomus orientalis; 2),Eurasian Jay (Garrulus glandarius; 1), Japanese White-eye (Zosterops japonicus; 1), unidentified buntings (Emberiza spp.; 1), White Wagtail(Motacilla alba; 1), Great Spotted Woodpecker (Dendrocopos major; 1), Great Cormorant (Phalacrocorax carbo; 1), Hodgson’s Hawk-cuckoo(Cuculus fugax; 1), Common Cuckoo (Cuculus canorus; 1), Juvenile Common Cuckoo (Cuculus canorus; 1), Yellow Bittern (Ixobrychus

sinensis; 2), Striated Heron (Butorides striata; 1); l brown rat (Rattus norvegicus; n 5 430), unidentified rats (9); m Includes: Ussuri white-toothed shrew (Crocidura lasiura; n 5 11), Eurasian red squirrel (Sciurus vulgaris; 4), lesser Japanese mole (Mogera wogura; 2), domestic cat(Felis catus; 1); n unidentified beetles (Calosoma spp.; n 5 4), locust (Locusta migratoria; 3), narrow-winged praying mantis (Tenodera

angustipennis; 1). u pea pebble crab (Philyra pisum; n 5 1), De Haan’s shore crab (Chiromantes dehaani; 1).

404 SHIN ET AL. VOL. 47, NO. 4

We obtained 1458 samples (924 pellets and 534prey remains) from 34 Eagle-owl territories: 429samples in the SRA and 1029 samples in the PGG.We identified a total of 1559 prey and 61 species (47birds, 10 mammals, two insects and two crustaceans;Table 2). For the SRA, we identified 29 bird species,representing approximately 73% of 41 identifiedprey species. The mean number of identified preyper territory was 45.9 6 7.1 (range 5 3–183) intotal, 34.0 6 5.1 (range 5 7–55) for the SRA and51.5 6 10.1 (range 5 3–183) for the PGG, which didnot differ between the two study areas (Mann-Whit-ney U-test, U 5 111.5, z 5 20.552, P . 0.05).

Diet composition differed between the two studyareas (Table 2), bird species being more commonthan mammals in the SRA. In contrast, mammalswere detected more frequently than birds in thePGG. Occurrence frequency of rodents of theSRA (.25%) was markedly lower than that of thePGG (,60%). On the contrary, ducks were detect-ed four times more in the SRA than in the PGG.Oriental Turtle-doves (Streptopelia orientalis), com-prising 19% of the total number of prey items, werethe most abundant prey species in the owl diet ofthe SRA.

The frequency of occurrence of the main preycategories varied significantly between the two studyareas (x2 5 246, df 5 11, P , 0.001). Partitioning ofthe contingency table and analysis of residuals indi-cated that, compared to a chance distribution, Mu-ridae were significantly underrepresented, and Ana-tidae and Rallidae were overrepresented in the SRA.

The mean Shannon’s diversity index (H9), food-nichebreadth (B) and standardized food-niche breadth (Bst)

at the species level for each territory differed significant-ly between the SRA and the PGG (Table 3). Shannon’sdiversity indices were greater in the SRA than in thePGG. Food niche-breadth of the SRA was about twotimes broader than that of the PGG. The value of stan-dardized food-niche breadth (Bst) of the SRA was alsohigh (.0.6), whereas that of the PGG was intermediate(0.2–0.4). Thus, the eagle-owls of the SRA captured rel-atively diverse prey.

Overall MWP was 300.2 g 6 9.17 (range 5 1–3400 g, n 5 1559). Birds were the most importantprey in biomass (approximately 73%; Table 2). An-seriformes, Galliformes, and Columbiformes madeup 86.0% of the bird biomass. In contrast, mammalswere less important in terms of biomass contribu-tion to the diet (27.6%). MWP differed significantlybetween the two study areas (Table 3) and was twotimes greater in the SRA than in the PGG. For theSRA, wetland-dependent prey composed 29.4% ofthe diet by frequency and 52.2% by biomass, andamong them, ducks made up .38% of the biomass.Ring-necked Pheasants (19.4%) made up the second-greatest biomass in the owl diet in the SRA. Rodentscontributed ,7% of the biomass in the diet in theSRA, but .30% of the owl diet in the PGG.

Individual regression analyses using percentagesof land-cover types within 1 km of the nest indicat-ed that the strongest predictor of MWP at a terri-tory was the percentage of wetland within that plotcircle (Fig. 2A): 57.2% of variation in MWP wasaccounted for by percentage of wetland. Thenext-best predictors of MWP were the percentagesof agricultural land and human settlements, whichexplained 38.4% and 29.6% of the variation in

Table 3. Trophic structure of Eurasian Eagle-Owls in wetland habitats (SRA study area) and non-wetland habitats (PGGstudy area) in west-central Korea during the period from September 2010 to August 2011 (n 5 34 territories overall).Territories with n , 20 prey items were excluded from the regression, and sample sizes are in parentheses. The P-valuesindicate the significance of the difference between the dietary indices in the two study areas. Values shown are mean6SE. For statistics, * 5 z, ** 5 t.

SRAa PGGb STATISTICS P

MWPc (g) 465.61 6 24.68 (374) 247.98 6 8.68 (1185) 29.858* ,0.001H’d 2.16 6 0.17 (11) 1.60 6 0.10 (23) 22.953** 0.006Be 8.24 6 0.89 (11) 4.18 6 0.35 (23) 23.590* ,0.001Bst

f 0.63 6 0.05 (11) 0.43 6 0.04(23) 22.837** 0.008

a Sihwa Reclaimed Area.b Paju City, Gimpo City, and Ganghwa District.c MWP 5 mean weight of prey (Mann-Whitney U-test).d Shannon’s diversity index (t-test).e food-niche breadth (Mann-Whitney U-test).f standardized food-niche breadth (t-test).

DECEMBER 2013 SPATIAL VARIATION OF EAGLE-OWL DIET 405

Figure 2. Relationship of mean weight of prey (MWP) with the percentage of (A) wetlands and (B) agricultural land inthe land surrounding eagle-owl territories in Korea during September 2010 to August 2011. The percentages of land-cover categories were arcsine-transformed, and 10 territories with fewer than 20 identified prey were excluded from theregression (remaining n 5 24). The equations for Fig. 2A and 2B are y 5 372.611x + 263.054 (r2 5 0.591, P , 0.001) andy 5 2375.664x + 459.545 (r2 5 0.41, P , 0.001), respectively.

406 SHIN ET AL. VOL. 47, NO. 4

MWP, respectively, but were negatively related toMWP (Fig. 2B).

Because the predictive power of the individuallinear regressions was relatively poor, we used a step-wise multiple linear regression analysis, which indi-cated that the percentages of wetland and agricul-tural land were the significant predictors (Table 4).These two variables together accounted for .66%of the variation in MWP (r2

adj 5 0.663). The stan-dard error of the estimate also decreased approxi-mately 30%.

DISCUSSION

Our results confirmed the trophic plasticity ofEurasian Eagle-Owls, which have varied diets corre-sponding to local prey availability and profitability.The diet composition, diet diversity indices, andMWP differed markedly between the two study ar-eas. In particular, the strong dependence on avianprey in the SRA differed from results of many pre-vious studies, in which eagle-owls ate primarily mam-mals (Konig et al. 1999, Martınez and Zuberogoitia2001, Marchesi et al. 2002, Penteriani et al. 2002,Lourenco 2006). Large size, high abundance, andyear-round availability of waterbirds in the SRA like-ly contribute to the abundance of birds in the eagle-owl diet.

Similar results were reported by Penteriani et al.(2012) for the eagle-owl diet in the wetlands of theDonana National Park (southwestern Spain), wherewaterfowl (7.0% of consumed biomass) representedthe second-most important prey group, just afterlagomorphs, and followed by Gruiformes (5.9% ofbiomass). However, even in that study, the bulk ofdiet (by biomass) was composed of mammals, whichcontrasts with eagle-owl diet in the wetland-domi-nated SRA in Korea, where avian prey made up.85% of biomass (Table 2).

In this study, eagle-owls of the SRA fed on wetland-dependent avian prey, primarily ducks, and Ring-necked Pheasants, which inhabited fields adjacent

to the nest sites. A variety of waterbirds such asspot-billed ducks, egrets, and Common Coots (Fulicaatra) breed within and around the wetlands of theSRA in spring and summer. In addition, a great num-ber of ducks visit the SRA as migrants in autumn andwinter (Park et al. 2009). The foraging habits of theEurasian Eagle-Owls here likely reflect the differenc-es of the land-cover types: the dependence on birds islikely linked to the proximity or prevalence of wet-lands in the SRA. As Murphy (1997) suggested, someowls occasionally rely more on alternative prey, andlow rat availability may result in a more diverse dietand higher predation on birds (Marchesi et al. 2002).In addition, lagomorphs, another staple prey of Med-iterranean eagle-owls (Delibes and Hiraldo 1979,Lesne and Thevenot 1981, Martınez and Zuberogoi-tia 2001, Martınez and Calvo 2001, Lourenco 2006)are scarce in west-central Korea. Korean hares (Lepuscoreanus) composed only 1.0% by number and 5.1%by biomass in SRA, and 0.3% and 2.4%, respectively,in PGG (Table 2).

Oriental Turtle-doves were also an importantfood resource of eagle-owls in west-central Korea,regardless of land-cover types. They commonly in-habit almost all regions, including forests, openlands, and agricultural lands (Lee et al. 2002). Thus,dove abundance in the diet of owls in both studyareas seems to be related to their abundance andease of accessibility.

As expected, the percentage of wetlands in theplot circles showed the strongest and positive rela-tionship with MWP (Table 4, Fig. 2A). This was like-ly due to the availability of large-sized, profitableprey in wetlands. On the contrary, the percentagesof agricultural land and of human settlements werenegatively related to MWP (Table 4, Fig. 2B), andthese land-cover types may harbor mostly smallerprey.

We suggest that the conservation value of wet-lands is increased by the fact that they may repre-sent a source of large and profitable prey for pred-

Table 4. Relationship between land-cover types and mean weight of prey in eagle-owl territories in Korea duringSeptember 2010 to August 2011, analyzed by a stepwise linear multiple regression. The percentages of land-covercategories were arcsine-transformed, and 10 territories with fewer than 20 identified prey items were excluded fromthe regression (remaining n 5 24). r2 5 0.692; r2

adj 5 0.663; SE 5 72.257; F 5 23.63; P , 0.001.

PREDICTOR COEFFICIENT STD. ERROR BETA t STATISTICS P-VALUE

Constant 349.933 37.731 9.274 ,0.001Wetland 291.043 66.351 0.600 4.386 ,0.001Agricultural land 2211.316 80.253 20.360 22.633 0.016

DECEMBER 2013 SPATIAL VARIATION OF EAGLE-OWL DIET 407

ators like Eurasian Eagle-Owls. We recommend fur-ther study on the relationships between eagle-owldiet and reproductive success in wetlands.

ACKNOWLEDGMENTS

We thank the Korean Cultural Heritage Administration,Ganghwa District, Gimpo City, Ansan City, Paju City, andHwaseong City for permission to conduct research and theKorean Ministry of Environment for support of GIS data.We are also grateful to Jong-In Choi and Hyeon-Chil Shinfor valuable help with fieldwork. Special thanks to Dr. JariValkama and two anonymous referees for critical review ofthe manuscript.

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Received 22 January 2013; accepted 28 June 2013Associate Editor: Vincenzo Penteriani

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