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Sc. Annals of DDI Tulcea, Romania
vol. 14 2008
SCIENTIFIC ANNALS OF THE DANUBE DELTA INSTITUTE, TULCEA – ROMANIA, 2008 69
Food habits of the Eurasian Eagle Owl (Bubo bubo) in Cheile
Dobrogei Gorge
SÁNDOR D. Attila
1
, BUGARIU Sebastian
2
1
- Environmental Protection Agency, 10 Podeni street, Targu Mures, RO-540253, Romania
2
- Societatea Ornitologică Română, 49 Gh. Dima street, ap. 2, Cluj Napoca, RO-400336, Romania
Address of author responsible for correspondence: Attila D. Sándor - Environmental Protection Agency, 10 Podeni street, Targu Mures,
RO-540253, Romania; e-mail: adsandor@gmail.com
BSTRACT. The first detailed feeding record of the Eurasian Eagle Owl (Bubo bubo) in Cheia Gorge, the Casimcea plateau,
Romania is reported. The diet contained mammals (51.41 %), birds (43.66 %) and amphibians (4.93 %). A total of 11 mammal
species were recorded, comprising representatives of four orders (Insectivora, Rodentia, Lagomorpha, Carnivora). Rodents
dominated the diet spectrum (42.56 % of all consumed prey), with the Romanian Hamster (Mesocricetus newtoni) being the
most frequent (22.5 %), followed by the Sibling Vole (Microtus epiroticus) and the Mound-building Mice (Mus spicilegus). Mammals
comprise 40.1 % of the consumed biomass. From biomass point of view the most important species is the Romanian Hamster (16.2 %),
but the East-European Hedgehog (Erinaceus concolor) and the European Hare (Lepus europaeus) have an important share in the diet,
too. The bird food is much more diverse; altogether 27 species were recorded, representing a number of eight orders. The bird
component of the diet is the most important in terms of biomass (59.2 %). Besides the pigeons and doves (35.4 %) as well as waterfowl
(8.10 %) the Galliformes (5.4 %) and the songbirds (5.1 %) are to be mentioned.The most important taxonomic group is the
Columbiformes (18.3 % of MNI), represented by only three species. The House Pigeon (Columba livia domestica) is the most used bird
species (12.7 % of MNI). Smaller mammals, other birds and amphibians made up a small portion of the diet in terms of biomass. The
diet composition is compared to similar studies, highlighting the importance of the steppe habitat and prey species’ diversity for the
maintenance of this owl population. The results suggest that the Eurasian Eagle Owl is an opportunistic feeder, capable to exploit also
species well adapted to steppe environments.
Key words: diet, Eurasian Eagle Owl, Bubo bubo, Cheia, steppe, Mesocricetus newtoni, Romania
INTRODUCTION
The Eurasian Eagle Owl is the largest owl in the world (1500-3500 g), with a wide distribution range across Europe, Asia
and North Africa [25]. It occurs in a variety of habitats, from boreal forests to Mediterranean scrublands, from steppes to
rocky gorges and riverside loess walls or sandy deserts. Typically it is an open landscape hunter and it uses a wide
range of food sources, from arthropods, fish, reptiles, amphibians to birds and mammals. It is the largest nocturnal avian
predator and its most common preys in the southern part of its distribution are medium to large sized mammals [4, 18, 19]
and medium to larger sized birds in the northern part. In Mediterranean landscapes it is the most important predator of
the Rabbit (Oryctolagus cuniculus), and it reaches peak abundances in regions where rabbits are widespread and
common [10, 11, 31].
Its diet and food selection are the targets of numerous studies and there is a certain scale and diversity found among the
diet spectra of owls from different landscapes and regions. The species’ feeding regime was intensely studied in its
Mediterranean breeding range [3, 10, 31], in Central [4, 8, 9] and Northern Europe [13, 20, 26]. Studies are much scarcer
in South East and Eastern Europe, yet in most countries surrounding Romania there are a number of studies detailing
eagle owl ecology and diet selection [16, 19, 24, 28, 29, 37]. Although the species is breeding in Romania, with certain
nesting locations known for decades [17, 30], its food was scarcely reported, usually only as short notes relating
noteworthy observations [1, 7, 12, 38]. There are only two detailed studies which treat the diet in Romania [32, 34].
Popescu and Sin [32] gave some details about the feeding regime based on pellets collected in three different sites in
Dobrogea, SE Romania, concentrating on sites in a mountain region (Măcin) and on two sites bordering wetlands, giving
a summary table of the major prey groups’ occurrence, without paying attention to the importance of individual prey
species. The other study was performed in a forested hilly landscape in the central part of the country, detailing a large
number of prey remains from two nesting sites [34]. Thus, in the steppe region of Romania the diet selection of Eurasian
Eagle Owls is virtually unknown.
Below we present the first detailed analysis of the diet of a breeding pair based on pellets and food remains collected
from nesting and roosting sites in Cheile Dobrogei Gorge in the central part of Dobrogea, in a typical short grass steppe
surrounded by agricultural fields.
STUDY SITE AND METHODS
10
.
A
Sc. Annals of DDI Tulcea, Romania
vol. 14 2008
SCIENTIFIC ANNALS OF THE DANUBE DELTA INSTITUTE, TULCEA – ROMANIA, 2008 70
The food composition of the Eurasian Eagle Owl was studied based on pellets and prey remains collected below rock
crevices in a small cliff system of the Cheile Dobrogei Gorge, Constanta County, South-East Romania. The site (see Fig.
1.) is a limestone gorge, here one pair bred in 2002 (Baczó Z. pers. com.), although the debris indicated a much longer
period of nest site use. The habitat surrounding the roost site is typical grassland in E, S, N and SW, and agricultural
areas used for wheat, corn, sunflower and oil-seed rape in the NW. The agriculture areas are smaller stands, in intensive
use with interspersed small grasslands and abandoned croplands as fallows. The grasslands are in extensive use for
grazing, cattle, goat and sheep are the most important species. There are some forested patches in the gorge, usually
small surfaces (a few ha) with dense lime (Tilia sp.) and hornbeam (Carpinus orientalis) thicket interspersed between
bare rocky surfaces. A small stream is crossing the gorge (Casimcea), with permanent water flux. The grasslands are
typical for the steppe region with Stippa sp. and Artemisia species being the dominant components. Sward height is low
(8-15 cm) on the hilly areas and much higher in grasslands inside the gorge (20-45 cm). The Cheile Dobrogei Gorge is a
protected area (geological and botanical reserve) and also part of two Natura2000 sites.
Fig. 1. The study area.
Diet analysis
The pellets were collected in April 2007 below the rock crevices used by owls for breeding and roosting. The material
collected was from nest debris and old, decomposed pellets. All material was collected, than it was sewed and the bones
of vertebrates were individually removed. For the identification of prey species we used the skulls and mandibulae,
humeri and tibia for mammals, humeri and tarsometatarsus for birds and skull bones for amphibians. Mammal and bird
remains were compared to the reference collection of A. S. Paired elements of each taxon were separated and the
largest number of elements was considered the minimum number of individuals (MNI) recovered from each sample. The
number of humeri and tibia was used for calculating MNI in case of missing skull for birds and for some mammals (only
in the case of rats Rattus sp., the Romanian Hamster (Mesocricetus newtoni) and the East-European Hedgehog
(Erinaceus concolor). Live biomass (B) was estimated using direct mass measurements of birds captured for banding
Sc. Annals of DDI Tulcea, Romania
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SCIENTIFIC ANNALS OF THE DANUBE DELTA INSTITUTE, TULCEA – ROMANIA, 2008 71
close to the study region (Sándor unpublished data), and also bibliographic sources [6, 14]. The mean prey weight (MPW)
was calculated summing the product of the prey number and mean body weight and divided by the number of prey items.
We analysed the food selection by using:
1. The mean prey weight (MPW) was calculated summing the product of the prey number and mean body weight
and divided by the number of prey items;
2. For diversity of prey species (H) we used the formula of Shannon and Wiener: H = -Σp
i
lnp
i
- where p
i
is the
proportion of any given prey species [3, 21];
Due to the high heterogeneity and fragmentation of the remains the results may be biased towards underestimating the
occurrence of small prey (shrews, smaller passerines, insects etc.). However, we suppose that this bias is of a lower
importance taking into account the low importance of these categories in terms of biomass and the usual low occurrence
in the diet of Eurasian Eagle Owl [23]. Being a larger bodied predator, the Eagle Owl is specialized in hunting medium to
large bodied mammalian and bird prey, thus it does not favour small prey species [4, 5, 18, 19].
RESULTS
The results of the analysis are presented in Table 1. A total of 142 individual prey remains were identified. Although the
number of prey items is low, the diet has a high species diversity (H = 2.19), with a total number of at least 41 species,
representing 11 mammal, 27 bird and at least 3 amphibian species. Mammals constituted 50.4 % of the diet, birds (62
individuals belonging to 8 orders) 43.6 % and amphibians only 4.9 %. The Romanian Hamster was the most common
prey in terms of number (22.5 % of MNI), followed by the Sibling Vole (Microtus epiroticus) as well the Mound-building
Mice (Mus spicilegus). Among mammals, the group of rodents is the most important for prey (42.5 % MNI), followed by
Carnivora, Insectivora and Lagomorpha. Birds are widely used by the owls, with eight different taxonomic groups
consumed (Gallo-Anserines, Falconiformes, Charadriiformes, Gaviiformes, Gruiformes, Strigiformes, Columbiformes and
Passeriformes), belonging to four different ecologic groups (grassland/steppe areas, forest, waterfowl and urbanized
species). Thus the most important group counted in number of species (14) and occurrences (5.6 %) is the group of
forest dwelling birds, although this is the least important in terms of biomass (1.4 %). The most important group in terms
of consumed biomass (5.4 %) is the group of urbanized species, the pigeons, doves and sparrows. Another group, with
high species number, occurrence and biomass share is the group of diurnal and nocturnal raptors.
The group of medium to large bodied prey has an important share in diet. Thus, the share of Hedgehogs, Water Voles
(Arvicola terrestris), Feral Pigeons (Columba livia domestica), Hares (Lepus europaeus) and Common Blackbirds
(Turdus merula) is above 1 %. All the other categories were represented by less than 20 individuals and counted less
than 1 % of the diet. From the biomass point of view the birds are the most important, with 59.2 %, followed by mammals
40.5 %, while the amphibians constitute 0.32 %. The Feral Pigeon is the most important species in terms of biomass
(27.3 %), followed by the Romanian Hamster (16.2 %), the East-European Hedgehog (8.1 %) and the European Hare
(Lepus europaeus).
Prey mass ranged from 10 g (Pelobates fuscus) to 1500 g (hares, martens and mallards), although prey < 50 g
accounted for less than 30 % of the diet. Prey species above 200 g account for 34.5 % of occurrence and 73.5 % by
biomass. The mean prey weight is 208.9 ± 162.0 g.
DISCUSSION
The Eurasian Eagle Owl is the largest nocturnal avian predator in Europe, distributed from the Mediterranean regions to
the tundra region, from seaside cliffs to boreal and alpine regions. It utilizes a wide range of food sources, usually
medium to large sized vertebrates [5]. The sample collected from the steppe region of Dobrogea is a small but diverse
one, representing a snap-shot of the abundance of available prey species in the region. The sample diversity
(represented by the Shannon-Wiener diversity index) is high and is made up by a wide range of species, from small to
large prey species. Although more than 40 species were recorded in the diet, two species dominate; these are the Feral
Pigeon and the Romanian Hamster. The Feral Pigeon is a common breeding species of caves and holes in the gorge,
abundant even in the rock crevices surrounding the owls’ roost (Sándor A. unpublished). The Romanian Hamster seem
to be the most important mammalian prey for the Eurasian Eagle Owl in Dobrogea, as it is a dominant prey in both
studies performed until now in Dobrogea [32, this study]. The Romanian Hamster is a European endemic species,
distributed on the western shores of the Black Sea, primarily in Dobrogea, both in its Bulgarian and Romanian part [27].
The species used to be considered a common pest in several agricultural areas in the second part of the XX
th
. century [2,
15, 22], but still it is one of the most understudied mammalian species of Europe [27]. Recent distribution and habitat
requirements are unknown, the only information about its abundance or distribution in the last 30 years are two food
selection papers [33, 35]. Hamster species are common prey of eagle owls all over their range [19], their high occurrence
Sc. Annals of DDI Tulcea, Romania
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SCIENTIFIC ANNALS OF THE DANUBE DELTA INSTITUTE, TULCEA – ROMANIA, 2008 72
was noted in the prey of Eagle Owls in Braşov, although there a different species was found, the Common Hamster
(Cricetus cricetus) [34].
Two more species are important as overall biomass components, the East-European Hedgehog and the Common Hare
(Lepus europaeus), both species being common prey species for the Eurasian Eagle Owl in most European studies [5, 8,
37].
Table 1.
Prey species in the diet of Eurasian Eagle Owl (Bubo bubo) in Cheile Dobrogei Gorge.
Species MNI %MNI %B
Erinaceus concolor 3 2,11 7,08
Talpa europaea 1 0,70 0,10
Mus spicilegus 8 5,63 0,40
Mesocricetus newtonni 32 22,54 16,18
Apodemus sylvaticus 2 1,41 0,13
Rattus norvegicus 2 1,41 1,35
Rattus sp. 4 2,82 1,62
Microtus epiroticus 9 6,34 0,73
Microtus sp. 3 2,11 0,24
Mustella putorius 2 1,41 2,70
Mustella nivalis 3 2,11 0,51
Martes sp. 2 1,41 2,70
Lepus europaeus 2 1,41 6,74
MAMMALIA 73 51,41 40,48
Podiceps ruficollis 1 0,70 0,40
Anas crecca 1 0,70 1,35
Anas sp. 2 1,41 4,05
Fulica atra 1 0,70 2,70
Accipiter nisus 1 0,70 1,35
Falco tinnunculus 2 1,41 0,94
Perdix perdix 1 0,70 1,01
Coturnix coturnix 1 0,70 0,37
Phasianus colchicus 2 1,41 4,05
Philomachus pugnax 1 0,70 0,37
Athene noctua 1 0,70 0,47
Asio otus 2 1,41 1,69
Columba palumbus 2 1,41 4,05
Columba livia domestica 18 12,68 27,30
Streptopelia decaocto 6 4,23 4,05
Melanocorypha calandra 1 0,70 0,15
Turdus merula 2 1,41 0,54
Turdus philomelos 1 0,70 0,20
Lanius collurio 1 0,70 0,08
Garrulus glandarius 1 0,70 0,27
Pica pica 1 0,70 0,30
Corvus monedula 3 2,11 1,21
Corvus cornix 2 1,41 1,35
Sturnus vulgaris 2 1,41 0,47
Carduelis carduelis 1 0,70 0,05
Carduelis chloris 1 0,70 0,10
Passeriformes sp. 4 2,82 0,34
AVES 62 43,66 59,20
Rana sp. 5 3,52 0,25
Pelobates fuscus 1 0,70 0,03
Bufo viridis 1 0,70 0,03
AMPHIBIA 7 4,93 0,32
TOTAL 142
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SCIENTIFIC ANNALS OF THE DANUBE DELTA INSTITUTE, TULCEA – ROMANIA, 2008 73
Birds constitutes and important component in the food of Eurasian Eagle Owl, especially in the Northern European forest
breeding populations. The most important groups in the food of owls are medium to large sized Columbiformes,
Galliformes, waterfowl, nocturnal raptors and large sized Passeriform birds (Corvidae and Turdidae). In our case the
pigeons and doves have a higher share in the diet; all the other groups are represented by a few individuals. All but two
species are common breeding or wintering birds in the region (Sándor A. unpublished). The two rare species are wildfowl,
preyed upon by owls most likely while migration. The third vertebrate group occurring in the diet of owls is made up by
the amphibians, three species were found. The Casimcea River crossing the Cheile Dobrogei Gorge is one of the only
three permanent rivers of Dobrogea, providing optimal habitat for amphibians around the breeding site of the owls.
Eurasian Eagle Owls consumed a varied diet in Cheile Dobrogei Gorge, indicating the utilization of a wide range of
microhabitats. Assessing the diet composition is often fundamental in wildlife management, especially when assessing
the impact of a predator on a conservation dependent species [36], or the importance of prey availability for a threatened
predator [9]. Differences in diet may be caused by local habitat differences, but also by changes in time occurred in the
population dynamics, the distribution and the density of important prey species. In conclusion, the knowledge of food
selection by the Eurasian Eagle Owl may be crucial not only to elaborate proper species conservation measures, but
also as an early warning tool in case of changes occurred in prey species populations.
Acknowledgements. We thank C. Dahlbeck, F. Kósa and A. Virginás for constructive comments on an early draft of the
manuscript and C. Domşa who provided help in the elaboration of the map in Fig 1.
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Manuscript received: February, 2008
Manuscript accepted: June, 2008
Printed: October 2008
