Original scientific article	UDC 589.293.1:591.53(497.4-14)
Received: 2009-08-20
THE DIET OF THE RAVEN CORVUS CORAX IN SOUTH-WEST SLOVENIA
Davorin TOME
National Institute of Biology, SI-1000 Ljubljana, Večna pot 111, Slovenia E-mail: Davorin.Tome@nib.si
Miha KROFEL
Department of Biology, University of Ljubljana, SI-1000 Ljubljana, Večna pot 111, Slovenia
Tomaž MIHELIČ DOPPS-Bird-Life Slovenia, SI-1000 Ljubljana, Tržaška 2, Slovenia
ABSTRACT
The diet of the raven was investigated in 2008 in south-west Slovenia. 158 food items, belonging to 26 different food groups, were retrieved from 60 whole and some broken pellets collected during the spring and autumn. Vertebrate remains were found in 57 pellets and plant debris in 53 pellets. Sheep remains were found in 32 pellets, and were significantly more common in pellets from sheep pastures than from areas with no pastures. Deer, invertebrate and bird remains were scarce.
Key words: the raven, Corvus corax, diet, south-west Slovenia
DIETA DEL CORVO IMPERIALE CORVUS CORAX IN SLOVENIA SUD-OCCIDENTALE
SINTESI
Gli autori hanno studiato la dieta del Corvo imperiale nel 2008 nella Slovenia sud-occidentale. 158 pezzi di cibo, appartenenti a 26 differenti gruppi alimentari, sono stati recuperati da 60 borre intere ed alcune danneggiate, raccolte durante la primavera e l'autunno. Resti di vertebrati sono stati ritrovati in 57 borre, mentre resti vegetali in 53 borre. Resti di pecora sono stati riscontrati in 32 borre, risultando significativamente piu comuni nelle borre raccolte sui pascoli ovini che in quelle raccolte nelle aree lontane dai pascoli. Resti di cervidi, invertebrati e uccelli sono risultati scarsi.
Parole chiave: Corvo imperiale, Corvus corax, dieta, Slovenia sud-occidentale
Davorin TOME et a/.: THE DIET OF THE RAVEN CORVUS COR$X IN SOUTH-WEST SLOVENIA, 161-166
INTRODUCTION
The raven population (Corvus coraxj consists of birds with two types of social behaviour: breeding birds, which appear to be territorial and live in pairs, and non-breeding, non-territorial birds, which live year-round in flocks (Ratcliffe, 1997). The raven population in Slovenia is estimated at 2000 to 3000 breeding individuals (1000 to 1500 breeding pairs; Geister, 1995) and at least 1500 to 3000 non-breeding individuals (Tome et a/., 2009). Although no quantitative data exists, there are some clues that the population has increased in recent decades (Tome et a/., 2009). This increase is consistent with observations all over Europe (BirdLife international, 2004; EBCC 2008) and is thought to be the result of the ban on shooting in some countries and increase of sheep farming practice in others (Tome et a/., 2009). The population size of ravens is known to correlate with the extent of sheep farming (Newton et a/., 1982).
The raven is first of all a scavenger and, as such, a vulture substitute in northern Europe (Ratcliffe, 1997). It feeds on dead animals from the size of a mouse to a whale. It can also be a predator, catching live prey. The extent to which it attacks live animals depends on the amount of available carrion. Rabbits are usually considered to be the biggest animals that ravens can successfully prey on, but then only if they are cornered or if their defence is disabled in some other way.
A common belief among farmers is that ravens kill sheep for food, especially lambs. The question of lamb killing (also calf killing) has been addressed by scientists several times (Ratcliffe, 1997; Langgemach et a/., 1995; Wallschlager et a/., 2004; Glandt, 2008), but only rarely hard evidence for successful predation was established. Even then, as found later, the animal under attack had ill developed internal organs or was affected by a potentially deadly disease. On the other hand, a lot of proof of ravens feeding on sheep carcasses has been established. Nevertheless ravens still have a bad reputation as large scale sheep killers.
The main aim of this work is to present the diet of the raven in south-west Slovenia, as derived from pellet analysis. Although raven diet studies are not rare, especially from England and Ireland (Marquiss et a/., 1978; Ewins et a/., 1986; Marquiss & Booth, 1986; Berrow, 1992), this is the first study of this kind from this part of Europe. It should be noted that the results cannot be taken as an arbiter of raven's guilt or innocence as a sheep killer, as they provide no information about predation and potential killing behaviour. Sheep remains in the raven's diet in this study are solely a sign of the bird feeding habits.
MATERIALS AND METHODS Study area
We collected pellets from two areas in south-west Slovenia. The first was located in the Karst region near the settlements of Sezana, Divaca and Kozina (squares on Fig. 1). Pellets were collected from and under the cliffs in rocky depressions, where raven nests were located. These localities were surrounded by forests or overgrown meadows and there were no pastures within at least a kilometre.
The second area comprised sheep pastures at Vo-lovja reber and Cure above Ilirska Bistrica (triangles on Fig. 1). We collected pellets under the trees and rocky outcrops within or close to pastures, where raven flocks were often spotted. We were not aware of any raven nests in this area. We estimate that pellets from both areas were not older than two months on the day of collection.
Materials and methods
The content of the pellets was analyzed in the laboratory. Pellets were soaked in water for ca. 15 minutes and then washed with tap water on a 0.5 mm mesh sieve. Material remaining on the sieve was dried and inspected macroscopically and, if needed, also under 100-400x magnification. The volume proportion of each food item found in a pellet was estimated visually. From each pellet we collected approx. 20 guard hairs and 5 feathers (when present), put them in 96% ethanol for approx. 30 minutes to clean, and let them dry. Macroscopic (general form, size, colour, rigidness, shape of the apex) and microscopic (structure and pattern of cuticular scales, shape of the cross-section, shape and size of the medullar space) characteristics of hairs were noted. The microstructure of the cuticle was inspected using cuticular imprints on celluloid plates. Species determination was performed according to Day (1966), Teerink (1991), Meyer ef a/. (2002) and with the comparative material stored at the Department of Biology at the University of Ljubljana. Teeth and bone remains of small mammals were identified according to Krystufek (1985, 1999) and with the comparative osteological material. Remains of bats were analysed by a specialist. Plant material was separated into green or wooden material, fruits and seeds. When possible we determined the plants more precisely.
Due to intensive digestion in the stomach, some food remains from pellets were in a very poor condition, and impossible to determine to a species level. For that reason the term food item in this text refers to the part of the material from one pellet belonging to a particular species, genus, family or class, whichever level of determination was the lowest possible.
Davorin TOME ei a/.: THE DIET OF THE RAVEN CORVUS CORAX IN SOUTH-WEST SLOVENIA, 161-166
postojna
dutovue
kozina
trst
ilirska s
bistrica'
Fig. 1: Study site (squares: localities of pellet collection on Karst region; triangles: localities of pellet collection on
pastures),
Sl. 1: Raziskovano področje (kvadrati: lokalitete zbiranja izbljuvkov na Krasu; trikotniki: lokalitete zbiranja izbljuvkov na pašnikih).
We used four methods to present pellet analysis results: 1) frequency of occurrence, calculated as the proportion of pellets in which a food item was found; 2) proportion of occurrence, calculated as the number of pellets where a particular food item was found divided by the number of food items from all pellets; 3) volume proportion, calculated as the sum of the volume proportions of a particular food item in pellets divided by the total number of pellets, and 4) average volume proportion, calculated as the sum of the volume proportion of a particular food item in pellets divided by the number of pellets including this food item only.
All pellets were analyzed by the same person (MK) to minimize variability caused by observer-related errors. We climbed the nests to collect the pellets only after the nestlings were fledged.
RESULTS
Food remains were analyzed in 60 whole raven pellets and some broken pellet material from 13 locations. Spring samples (15 pellets) were collected from 9th to 13th May 2008 in the Karst region only, autumn samples (45 pellets) from 11th September to 15th October 2008 in pastures. 158 food items, belonging to 26 different food groups, were separated from pellets. In single whole pellets we found one to five food items, on average 2.4 food items per pellet (s.d. = 0.85).
Vertebrate remains were found in 57 whole pellets (95%), and plant remains in 53 whole pellets (88%). Sheep remains were found in 32 whole pellets (53%), while remains of other domesticated animals were scarce. Small mammals were also relatively common, followed by deer, birds and invertebrates. Green plant material was the most common plant food item, followed by fruits and seeds (Tab. 1).
Davorin TOME et a!.: THE DIET OF THE RAVEN CORVUS CORAX IN SOUTH-WEST SLOVENIA, 161-166
Tab. 1: Diet composition of ravens in SW Slovenia (N of all food items is 158). Tab. 1: Sestava prehrane krokarjev v JZ Sloveniji (N vseh enot hrane je 158).
	N of food	Frequency	Proportion	Volume	Average volume
Food item	items	of occurrence	of occurrence	proportion	proportion
animals	78	0.919	0.494	0.548	0.436
Vertebrata	71	0.919	0.449	0.518	0.452
Mammalia	65	0.887	0.411	0.450	0.429
Cervidae	7	0.113	0.044	0.054	0.476
Carnivora	4	0.065	0.025	0.021	0.333
undet. large mammals	4	0.065	0.025	0.016	0.255
domesticated ungulates	36	0.581	0.228	0.228	0.393
sheep	32	0.516	0.203	0.224	0.434
goat	2	0.032	0.013	0.002	0.050
Pig	1	0.016	0.006	0.002	0.100
horse	1	0.016	0.006	0.001	0.050
small mammals	14	0.210	0.089	0.130	0.575
Myotis myotis	2	0.032	0.013	0.020	0.625
Muridae	2	0.032	0.013	0.010	0.325
Gliridae	2	0.032	0.013	0.021	0.650
Talpa europaea	1	0.016	0.006	0.015	0.900
undet. small mammals	7	0.113	0.044	0.064	0.564
Aves	6	0.097	0.038	0.068	0.703
Passeriniformes	3	0.048	0.019	0.034	0.700
Falconiformes	1	0.016	0.006	0.008	0.520
undet. birds	2	0.032	0.013	0.026	0.800
Invertebrata	7	0.113	0.044	0.030	0.269
Coleoptera	3	0.048	0.019	0.021	0.440
Saltatoria	1	0.016	0.006	0.008	0.500
undet. insects	2	0.032	0.013	0.000	0.015
snails	1	0.016	0.006	0.000	0.030
plants	67	0.806	0.424	0.433	0.401
green plant material	40	0.645	0.253	0.306	0.475
fruit	1	0.016	0.006	0.016	1.000
cereals	1	0.016	0.006	0.016	1.000
corn	2	0.032	0.013	0.014	0.425
undet. fruits and seeds	18	0.290	0.114	0.075	0.257
wooden plant material	5	0.081	0.032	0.006	0.078
stones	10	0.161	0.063	0.009	0.057
undetermined	3	0.048	0.019	0.009	0.193
Sheep remains were significantly more common in pellets found in the area with sheep pastures than in those in the area with no pastures. Ravens distant to pastures fed more on fruits, seeds, small mammals and large wild mammals. There was also a significant difference in the proportion of bird remains between two areas. Very few were found in pellets from pastures, but rather more in pellets found in areas where sheep were not available as food (Tab. 2).
DISCUSSION
None of the quantitative measures of the raven diet in this study (frequency of occurrence, proportion of oc-
currence, volume proportion) are directly proportional to the real quantities of food ingested. For example, sheep remains from pellets in one locality could have arisen from one or from several animals; ravens could have eaten a whole sheep or only a part of it. The amount of food remains regurgitated in pellets depends on the degree of digestion, which is different for different items. So the results do not provide an exact picture of the diet. They are probably quite accurate in presenting its qualitative composition, but only provide an approximate indication of the importance of particular food items.
Davorin TOME et a/.: THE DIET OF THE RAVEN CORVUS CORAX IN SOUTH-WEST SLOVENIA, 161-166
Tab. 2: Diets of ravens dwelling near and far from sheep pastures. Numbers are proportions of the occurrence in pellets, the difference is denoted using Chi-square test (* p< = 0.05; ** p < = 0.01).
Tab. 2: Prehrana krokarjev, živečih blizu in daleč stran od ovčjih pašnikov. Številke predstavljajo delež pojavljanja v izbljuvkih, razlika je označena s testom hi-kvadrat (* p< = 0,05; ** p < = 0,01).
Prey category	Sheep pastures far	Sheep pastures near	Chi-square
sheep	0.065	0.259	**
domesticated animal excl. sheep	0.109	0.027	*
large wildlife	0.130	0.045	
small mammals	0.152	0.063	
birds	0.109	0.009	**
invertebrates	0.065	0.036	
green plant material	0.109	0.313	*
fruits and seeds	0.174	0.125	
wood, stones, undetermined	0.087	0.125	
N food items	46	112	
Sheep carrion appears to be a frequent food item in raven pellets in south-west Slovenia, which is similar to findings of studies made elsewhere (i.e., Newton ef a/., 1982; Ewins ef a/., 1986). In our samples from all localities, sheep remains were found in over half of the pellets. But sheep are by no means essential for the survival of ravens. Almost no sheep remains were found in pellets collected from nest sites. This indicates that even at a time of great demand for food, when adults must feed themselves and the young birds, ravens can thrive without domesticated animals as a food source, at least if they have sufficient nutritious plant material, small mammals and birds at hand, which were frequently found in those pellets.
Although in two sampling areas pellets were collected in different seasons (pellets from the Karst region in breeding season pellets above Ilirska Bistrica in post-breeding season), we argue that differences in the diet are not season related. The raven is opportunistic in its diet, exploiting whatever locally is available the most (Ratcliffe, 1997). The difference is probably mostly due to lack of sheep pastures in one and its abundance in the other area.
There is little doubt that pellets from nest sites belong to breeding ravens, adults or young fed by adults. There is also no reason not to believe that pairs with nests close to pastures have more sheep remains in the diet
than those we inspected. Less clear, however, is the social status of ravens whose pellets were found in autumn at Volovja reber and Cure. Since they were observed in groups, not in pairs, it is possible that they belong to the year-round flocks of non-breeding individuals. According to Ratcliffe (1997), breeding birds in the UK remain around the nest site in pairs throughout the year, protecting the territory, so large groups can always be considered as non-breeders. More observations on individually marked birds or even telemetry studies would be necessary to investigate whether, in Slovenia, breeding pairs join non-breeders in flocks after the breeding season. The results would be interesting from the point of view of the raven - sheep interaction. They would add some understanding to the current enigma as to which social group is responsible for ravens being on farmers' black lists: non-breeders or both non-breeders and breeders - quite possibly only the more mobile non-breeders.
ACKNOWLEDGEMENTS
We thank Primoz Presetnik for determining bat remains and Dr. R. H. Pain for improving the English. Financial support was provided by the Environmental agency of the Republic of Slovenia.
Davorin TOME et al.: THE DIET OF THE RAVEN CORVUS CORAX IN SOUTH-WEST SLOVENIA, 161-166
PREHRANA KROKARJA CORVUS CORAX V JUGOZAHODNI SLOVENIJI
Davorin TOME Nacionalni inštitut za biologijo, SI-1000 Ljubljana, Večna pot 111 E-mail: Davorin.Tome@nib.si
Miha KROFEL
Oddelek za biologijo, Univerza v Ljubljani, SI-1000 Ljubljana, Večna pot 111
Tomaž MIHELIČ DOPPS-Bird-Life Slovenia, SI-1000 Ljubljana, Tržaška 2
POVZETEK
V letu 2008 smo v jugozahodnem delu Slovenije raziskovali prehrano krokarjev (Corvus corax) na osnovi preiskave izbljuvkov. Izbljuvke smo nabirali na dveh lokacijah, na Krasu in na območju Volovje rebri in Gur pri Ilirski Bistrici. Iz 60 celih in nekaj razdrobljenih izbljuvkov smo izolirali 158 enot hrane, ki so pripadale 26 različnim taksonomskim skupinam. V 57 izbljuvkih smo našli ostanke vretenčarjev, v 53 izbljuvkih ostanke rastlinske hrane. Ostanke ovc smo odkrili v 32 izbljuvkih; značilno več v izbljuvkih nabranih v bližini ovčjih pašnikov kakor v izbljuvkih nabranih na lokacijah, ki so bile od pašnikov oddaljene. Ostanki srnjadi, jelenjadi, ptic in nevretenčarjev so bili redki.
Ključne besede: krokar, Corvus corax, prehrana, jugozahodna Slovenija
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