An attempt to determine the size of the Common Barn-owl’s ( Tyto alba ) hunting area based on its prey composition

Abstract

The relative abundance of small mammal species detected from Common Barn-owl pellets reflects the landscape structure and habitat pattern of the owl’s hunting area, but it is also affected by the size of the collected pellet sample and the size of the supposed hunting area. The questions arise: how many pellets should be collected and analyzed as well as how large hunting area should be taken into consideration in order to reach the best correspondence between the owl’s prey composition and the distribution of habitats preferred by small mammals preyed in supposed hunting areas? For this study, we collected 1045 Common Barn-owl pellets in a village in southern Hungary. All detected small mammal species were classified into functional groups (guilds) preferring urban, open, forest and wetland habitats. The proportion of functional groups was compared to the proportion of these habitats around the pellet collection site within circles of one, two, and three km radius. Saturation curves showed that at least 300 pellets or ca. 600 mammalian remains are required for the detection of the 19 small mammal species. The share of small mammals detected in the prey and their functional groups according to their habitat preference showed an increasing consistency with the distribution of real habitats in the potential hunting area of a radius of 3 km around the owl’s breeding or resting place.

Keywords

Functional group habitat preference landscape pellet small mammal

Introduction

The breeding and resting places of Common Barn-owls (Tyto alba) are most often associated with human settlements, where pellets consisting of indigestible parts of their prey can be collected in large numbers.^1–3 Precious information on the occurrence and abundance of small mammal species in the owl’s hunting area can be collected on the basis of prey remains from pellets, therefore this indirect method is often used for fauna surveys.^3–5 In contrast to the trapping of small mammals, a method of pellet analysis is appropriate for the detection of rare and difficult-to-trap species, though a larger number of pellets is needed.^6,7 The owls’ prey composition is amongst other influenced by the seasonal dynamics of small mammals, such as population outbreaks as well as by the prey preference of owl individuals.^8–10 These distorting effects can be reduced by an appropriate pellet sample size. The Common Barn-owl usually occurs alone or in pairs, so it is not easy to collect a large number of pellets in a short time. If pellet collecting at the same site is not repeated, even for several years, the sample is unlikely to be representative. The question is still not finally answered: how large should the pellet sample be to be representative of the small mammal community in the area?^8,11–14

The landscape structure of the Common Barn-owl’s hunting area, i.e. the extent and pattern of habitat patches, changes in space and time mainly because of human impact. These changes can be traced through the prey composition of owls due to the habitat preference of small mammals.^7–9,15,16 The results of such research may also be influenced by the size of the owls’ hunting area, which, in most cases, is an assumed value, an area around the owls’ breeding or resting place, usually described by circles with a radius of 1–3 km.^6,15–31 Rodríguez and Peris³² considered the boundaries of settlements, an average radius area of 3.6 km as a unit for the study of the relationship between small mammals and landscape structure. Movement areas corresponding to the area of circles with radii of four and 5.6 km were taken into account by Martínez and Zuberogoitia.³³ In the Balearic Islands, it has been found that Common Barn-owls also fly over the 4.5 km wide strait for hunting to catch prey on the neighboring island.³⁴ According to a study performed in Canada, owls move 8–10 km from their regular roost site for one night only.³⁵ Based on the described different approaches, it is still not clear how large an area might be that actually represents the Common Barn-owl’s hunting area.

The objectives of our study were to (1) determine the size of the Common Barn-owl pellet sample required to represent the small mammal community in the area, and (2) determine the size of the hunting area whose structure is most consistent with the composition of the small mammals prayed by the owl.

Methods

Common Barn-owl pellets were collected on 16.03.2017 in the attic of a house in Udvar, a village in southern Hungary (Figure 1). It is a small settlement with only 150 inhabitants near the Hungarian-Croatian state border, surrounded by agricultural lands (Figure 1). Its climate is moderately warm, moderately dry; with yearly average temperature of 10.0–10.8°C and an average annual precipitation fluctuating between 600 and 670 mm.³⁶

Figure 1.

Location of the sampling site in Udvar village (•) and structure of the surrounding landscape according to the CORINE map in the circle with 1 km, 2 km, and 3 km radius.

Single pellets were dissected by hand, using tweezers and a toothbrush. The number of small mammal specimens extracted from the pellets was given based on the number of skulls and related mandibles. The taxonomic determination of small mammals was performed on the basis of skull, mandible, and teeth parameters.^5,37–40

The whole sample of collected Common Barn-owl pellets (1045) was randomly divided into groups of 100. The small mammal species detected from the pellets were classified into four functional groups according to their habitat (urban, open, forest, wetland species) preference (Table 1). The saturation curve was plotted based on the cumulative value of the number of small mammal species detected from each group of hundred pellets. To determine the saturation curve of prey species, we used the Individual rarefaction method of the Past program.⁴¹

Table 1.

The number of specimens in the randomly sorted 100 pellet samples, the total number of specimens, and the relative abundance of the species (habitat preference: u – urban, o – open, f – forest, w – wetland).

No. of pellets		100	100	100	100	100	100	100	100	100	100	45	∑ 1045	%
Crocidura leucodon	o	7	7	6	11	4	10	11	8	1	19	2	86	3.77
Crocidura suaveolens	o	4	8	9	11	14	21	21	7	3	22	2	122	5.34
Sorex araneus	f	1	6	6	1	6	8	1	1	1	4	4	39	1.71
Sorex minutus	f	0	3	0	1	0	5	0	0	0	0	1	10	0.44
Neomys anomalus	w	1	0	0	2	5	2	4	1	0	6	0	21	0.92
Muscardinus avellanarius	f	0	0	1	2	0	0	0	1	0	1	0	5	0.22
Microtus lavernedii	w	1	7	1	0	6	0	1	2	0	2	1	21	0.92
Microtus arvalis	o	137	149	152	139	140	128	139	125	109	134	72	1424	62.35
Microtus subterraneus	f	0	0	3	0	0	1	0	0	0	0	0	4	0.17
Arvicola amphibius	w	1	3	1	2	1	0	2	0	0	1	0	11	0.48
Myodes glareolus	f	1	0	1	1	1	0	3	1	0	5	3	16	0.70
Cricetus	o	0	0	1	1	2	0	0	0	1	0	0	5	0.22
Apodemus agrarius	f	13	16	8	23	11	30	18	24	18	23	4	188	8.23
Apodemus flavicollis	f	4	4	5	9	6	9	4	3	1	11	4	60	2.63
Apodemus sylvaticus	f	5	4	3	4	0	4	7	4	5	4	2	42	1.84
Apodemus sp		2	1	3	2	3	4	2	5	7	2	1	32	1.40
Micromys minutus	w	1	1	0	1	1	0	1	0	1	2	0	8	0.35
Mus musculus	u	4	5	2	6	12	2	6	6	2	9	1	55	2.41
Mus spicilegus	o	0	4	6	7	4	4	3	4	2	6	0	40	1.75
Rattus norvegicus	u	6	1	6	5	3	3	6	7	2	4	2	45	1.97
Aves indet. (Hirundo sp.)		0	1	1	1	1	2	0	0	0	1	0	7	0.31
Pelobates fuscus		1	5	2	7	0	0	10	9	0	2	0	36	1.57
Anura (Rana sp.)		0	0	1	0	2	1	0	0	0	2	0	6	0.26
Coleoptera (Tenebrio sp.)		0	0	0	1	0	0	0	0	0	0	0	1	0.04
Total number of specimens		189	225	218	237	222	234	239	208	153	260	99	2284	100.00
No. of mammal species		14	14	16	17	15	13	15	14	12	16	12	19

We created 10 different random sequences of hundreds of pellet samples and then calculated the cumulative species numbers of small mammals by increasing the number of pellets. The saturation curve was generated from the mean and standard deviation of the obtained cumulative values.

To examine the landscape structure of the potential owl hunting area, we used 2012 maps of the CORINE database.⁴² Using QGIS,⁴³ we determined the distribution of CORINE land cover classes as landscape structure types on the maps. Habitat types of the studied landscape (Figure 1) were adjusted to the habitat preference of small mammals: urban (112 discontinuous urban fabric), open (211 Non-irrigated arable land, 231 Pastures, 242 Complex cultivation patterns and 243 Land principally occupied by agriculture, with significant areas of natural vegetation), forest (311 Broad-leaved forest and 324 Transitional woodland-schrub) and wetland (411 Inland marshes).

To determine the size of the potential hunting area, circles with radii of 1, 2, and 3 km around the owl pellet collecting site were designated. The share of the four habitat types was examined within the area of the three designated circles around the pellet sampling site (Figure 1). To determine the size of the hunting area whose structure is most consistent with the composition of the small mammals prayed by the owl, the comparison of the relative abundances of small mammal functional groups detected in the whole sample of pellets and the proportion of the habitats within the corresponding circles were carried out by a homogeneity G test.⁴⁴

Results

There were 2284 specimens of prey remains found in the 1045 Common Barn-owl pellets collected (Table 1). 97.99% of the prey was small mammals. One pellet contained an average of 2.14 mammals. In the whole sample of pellets 19 small mammal species were detected (Table 1).

The number of detected small mammal species increased with the size of the sample and at a sample of 300 pellets, it was close to reaching the maximum (SD 18 ± 0.89) (Figure 2.). This sample size corresponds to 642 specimens, calculated with 2.14 small mammals per pellet (Figure 3).

Figure 2.

Mean and standard deviation of the cumulative species number of small mammals in the ten times randomly sorted pellet samples.

Figure 3.

The change in the number of small mammal species detected from Common Barn-owl pellets according to the number of specimens based on the individual’s rarefaction.

Calculating the specimen rarity based on the number of specimens, the saturation curve reaches the plateau phase at about 600 specimens (Figure 3).

The share of functional groups formed on the basis of habitat preference of the 19 detected small mammal species differed the most from the distribution of their habitats within a radius of 1 km, only the relative frequency of species occurring in open and urban habitats was similar (Tables 1 and 2). Within a radius of 2 km, only the relative frequency of species preferring forest habitats differed from the proportion of their habitats (Table 2). Within a radius of 3 km, the relative frequency of the species belonging to the four functional groups and the distribution of their habitats did not differ significantly (Table 2).

Table 2.

Differences between the functional groups of small mammals detected from the whole sample of pellets of the Common Barn-owl, based on their habitat preference and the proportion of their preferred habitats within radii of 1, 2, and 3 km.

Habitat	Urban	Open	Forest	Wetland
Proportion of small mammals	4.45	76.23	16.09	3.23
Habitats proportion within a radius of 1 km	8.12	91.88	0.00	0.00
G	1.08	1.46	22.31	4.47
P	NS	NS	p < 0.001	p < 0.050
Habitats proportion within a radius of 2 km	2.14	92.92	1.87	3.07
G	0.83	1.65	12.88	0.00
P	NS	NS	p < 0.001	NS
Habitats proportion within a radius of 3 km	2.22	85.32	9.82	2.63
G	0.77	0.51	1.53	0.06
P	NS	NS	NS	NS

Discussion

Our study showed that the sample size of 300 pellets of Common Barn-owl is large enough to represent the small mammal fauna of its hunting area, and we have shown that there is little likelihood of detecting new small mammal species by further increasing sample size. Based on a high number of studies on the Common Barn-owl diet also performed in Hungary, Kalivoda¹² concluded that the content of 200 pellets is representative of the small mammal fauna of an area. However, Horváth¹³ based on his own study, suggests that further collecting of more than 100 pellets is meaningless, as the number, frequency, and diversity of prey species do not change significantly. Our results are similar to those of a study performed in Spain that showed that 300 to 500 Common Barn-owl prey items in lowland and 500–700 prey items in a mountainous area are needed to get a realistic picture of small mammal species occurring in an area.¹⁴ According to a study performed in North America, 800–1000 prey items must be determined in order to be sufficiently representative of the owl’s diet composition.¹¹ We note that these values were obtained from a large number of samples of different sizes collected from different locations with different features^11,14 whereas, in contrast, we drew our conclusions from a large sample from one location. Being an opportunistic predator, the Common Barn-owl consumes each prey species in proportion to their occurrence,^37,45 so the representative sample size may differ in different locations.

The proportion of functional groups of small mammals detected from pellets, which are formed according to their habitat preference, became more similar to the distribution of habitats as the size of circle as potential hunting area increased. The study area is agricultural landscape, the proportion of urban areas in relation to the size of all three potential hunting areas is small, and this was well reflected in the prey composition of owls by the low proportion of the urban small mammal species.

In our study area, the hunting area with a radius of 1 km around the resting area lacked the typical wetland and forest habitats, so the small mammals that preferred these habitats were probably preyed on by the owls at a greater distance. Our results confirm that circles with a radius of 1.5 km or less may only be suitable for characterizing the environment of the breeding site,³³ however, many researchers use it to characterize the hunting area of Common Barn-owls.^{20,21,23,25–27} It should not be overlooked that the range of motion of the Common Barn-owls is relatively small,^3,19,46 and cannot be described by a regular circle, but it follows irregularly shaped habitats most suitable for hunting.⁴⁷

When the supposed hunting area increased to a circle with a radius of 2 km, only the proportion of forest habitats differed significantly from the relative frequency of the small mammal species that preferred them. The large number of specimens of species that prefer forest habitats in the prey can be explained by the fact that the rows of trees and smaller groups of trees in the area provide them with suitable habitat at the edges of which they can be easily preyed on by owls.³ Arlettaz et al.⁴⁸ found that owls avoided forests due to the difficult availability of small mammals, but were more likely to hunt at forest edges. The habitat preference of small mammals in the prey of owls already better reflected the distribution of habitats within a radius of 2 km. In several previous studies, the area of circles with radii of two and 2.5 km was considered as the range of motion of the owl.^{6,16,17,22,24,29,30–31}

Based on our results, the composition of the small mammal prey of Common Barn-owls, taking into account the habitat preference of these species, was fully consistent with the distribution of preferred habitats within a radius of 3 km. Our finding supports the results that the hunting area of Common Barn-owls corresponded to a circle with a radius of 3 km or more.^10,18,32,33 Based on radio-telemetry studies, individual ranges of movement can be very different and the size of their hunting area may be determined by the size of pasture and cereal crop areas.⁴⁸ The Common Barn-owl hunts mainly in such open areas, as the abundance and availability of prey are optimal there.^47,48 According to Naim et al.,⁴⁹ the home range of owls increases significantly and they move further away from the nest if the number of potential prey species decreases due to the effect of rodenticides. The range size of Common Barn-owls varies widely, as it is significantly influenced by the distribution of habitats and the availability of prey species,³ therefore the size of the area to be studied and the size of the representative sample may vary depending on the site.

Based on the results of our study, we recommend collecting and processing at least 300 pellets from each roosting or breeding site for faunistic research or ecological studies in the Central European agricultural landscape, and a circle with a radius of 3 km around the pellet collecting site should be characterized as the hunting area of the Common Barn-owl.

Footnotes

Acknowledgements

The authors are very grateful to the editor and anonymous reviewers for the useful comments on the previous version of manuscript. We are also grateful to Csaba László and Béla Simonkovics for their help during the field work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was financially supported by PhD School of Biology and Sport Biology at the University of Pécs.

ORCID iD

Jenő J. Purger

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