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POLISH JOURNAL OF ECOLOGY (Pol. J. Ecol.) 61 1 179–181 2012 Research note Grzegorz KOPIJ Department of Vertebrate Ecology, Wroclaw University of Environmental & Life Sciences, Kożuchowska 5b, 51-631 Wroclaw, Poland, e-mail: [email protected] Department of Wildlife Management & Research, University of Namibia, Katima Mulilo Campus, Private bag 1096, Winela Rd., Katima Mulilo, Namibia, e-mail: [email protected] FOOD NICHE OVERLAP IN CO-EXISTING BARN OWL TYTO ALBA (SCOPOLI 1769) AND GREAT HORNED OWL BUBO VIRGINIANUS GMELIN 1788 IN INTESIVELY USED FARMLAND ABSTRACT: The study was conducted at a site where the Great Horned Owl and the Barn Owl nested in a close proximity to each other in an intesivelly managed farmland in north-eastern Oregon. By the means of pellet content analysis it has been shown that food niches of both species almost totally overlapped. Their main diet was composed of Microtus voles (numerically: 79% in Great Horned Owl and 86% in Barn Owl) which were probably superabundant in this site. KEY WORDS: diet, Tyto alba, Bubo virginianus, intensively used farmland, food niche, Oregon One of the central concept in ecology is the competitive exclusion principle, which explain species coexistence. In general this concept forbids the coexistence of two species with identical or similar food niches. However, some species have been shown to coexist on the same resources with no recorded evidence of niche differentiation (e.g. Kopij 2006). The quest for mechanisms permitting such coexistence may be of crucial importance in conservation biology, as competitive exclusion can influence the range, distribution and densities of species (B e gon et al. 1990). This coexistence may be possible through a combination of factors such as journal 33.indb 179 non-limiting food resources and a high rate of predation (Wo o dw ard and Hi l d re w 2002, Kopij 2006). It has been shown that food niche overlap between two competing species decreases as their body size increases (He re r a and Hi r a l d o 1976; Woodward and Hi l d re w 2002). It increases in summer, when prey are abundant and small, and decresases in winter when the prey is larger but scarcer (Wo o dw ard and Hi l d re w 2002). It also shows some geographical variations (He r rer a and Hi ra l d o 1976). Niche diffrerentiation is also not the only mechanism by which two competing species can coexist (B egon et al. 1990). In the Nearctic Region, the Barn Owl Tyto alba (Scopoli 1769) nests often co-occurs sympatrically with the Great Horned Owl Bubo virginianus Gmelin 1788 (Johnsgard 2002, Konig and Weick 2008). Under such condition, their diet was analysed under seminatural conditions, namely in a short-grass prerie mixed with farmlands in Colorado (Mar t i 1974), and semidesert vegetation mixed with a farmland in California (Rudoph 1978). Since both owl species differ significantly in body size (Great Horned Owl: mean weight – 1400 g; Barn Owl: mean weight – 500 g; Johnsgard 2002), significant differences 2013-04-30 10:05:52 180 Grzegorz Kopij in prey composition, mean biomass of prey consumed and proportions of particular prey species have been evidenced in both studies. This presented note contributes further to the issue of food niche segregation between the the Great Horned Owl and Barn Owl. In this case, however, studies were conducted in highly modified habitat, i.e. in intesivellymanaged farmland. The study aims to show to what extend the food niche of both owl species may overlap under such condition. The study was conducted at a site where the Great Horned Owl and the Barn Owl nested in a close proximity to each other (ca. 1 km apart), on a farm yard at the East Oregon Agricultural Research Station at Union, Union County, NE Oregon (45°12’36’’N, 117°51’53’’W). The farmstead comprised a wet pastureland grazed by horses, cattle, and deers, and intesivelly cultivated fields (mainly corn and wheat) further afield on one side, and well-timbered suburbs on another side. The diet composition was determined through pellet content analysis. Pellets were collected on 29 September 2010; 91 pellets from the Great Horned Owl and 189 pellets form the Barn Owl were collected (Table 1). Only compact, i.e. entire pelltes were taken into analysis. The pellets were analysed with universally accepted procedure (cf. Ya lden 2009). Identification of rodents and calculation of their biomass were based on Mas er and Stor m (1970) and Ver ts and C ar raway (1998). The other prey were identified with the aid of reference collection hold by the Department of Forestry of the Oregon State University. The following trophic estimators were used to characterize diet of both owl species: H’ – Shannon and Weaver’s prey species diversity index, FNB – Simpson’s food niche breadth index, O – food niche overlap Pianka’s index, and M – mean prey biomass (Shan non and We ave r 1949): H’ = - Σpilogepi FNB = 1/∑pi2 O = ∑ pi ∑qi / ∑ (pi2∑ qi2)1/2 where pi is the proportion of the total number of individulas which belong to the i-th species in one owl’s diet, and qi is the proportion of the total number of individulas which belong to the i-th species in the other owl’s diet. Both FNB and O were calculated on the fine resolution using prey species or genera. Rodents constituted the bulk of the summer diet both of the Barn Owl (98.4% by weight) and Great Horned Owl (91.8% by weight). The remaining prey were represented by birds (by weight 3.2% in the Barn Owl and 8.2% in Great Horned Owl diet). In the diet of the Barn Owl the medium-sized birds were represented exclusively by the Rock Pigeon Columba livia, which commonly nested in the barn occupied by the owls. Among rodent prey, voles Microtus spp. were by far the commonest prey in the diet of both owl species compared. The Northern Pocket Gopher Thomomys talpoides (R ichards on 1828) supplemented the rodent diet. Other rodents, such as Deer Mouse Pero- Table 1. Summer diet of the Great Horned Owl and Barn Owl at Union, NE Oregon. F – percentage of the frequency of occurrence, N – the percentage of prey items, W – the percentage of the approximate biomass (g). F Bubo virginianus N W F 69 19 9 1 79 13 6 1 64 21 3 4 73 22 2 1 - 86 10 1 1 - 78 18 1 <1 - 2 - 1 - 8 - 2 1 1 <1 3 1 91 141 7290 189 328 15312 Prey taxa Mammalia Microtus spp. Thomomys talpoides Peromyscus maniculatus Mus musculus Sciurus spp. Aves Medium-sized birds Small birds Number of pellets/prey items/biomass (g) journal 33.indb 180 Tyto alba N W 2013-04-30 10:05:52 Food niche overlap Tyto alba and Bubo virginianus myscus maniculatus (Wag ner 1845), House Mouse Mus musculus L., 1758 and squirells Sciurus sp. were preyed upon occassionally (Table 1). Although the mean prey biomass was higher in the Great Horned (59.3 g, N = 123) than in the Barn Owl (46.7 g, N = 328), the index of prey species diversity was very similar, i.e. H’ = 0.67 for the Great Horned Owl and H’ = 0.52 for the Barn Owl. Food niche breadth index was much the same for both owl species (FNB = 1.54 and 1.33 for the Great Horned and the Barn Owl, respectively). The index of food niche overlap on fine resolution was O = 0.93, which means that the food niches almost totally ovelapped. The high similarity of the Barn Owl and Great Horned Owl diet nesting in the same site was rather unexpected as the owls differ both in size and in behaviour (Jonsg ard 2002). This high-degree overlap in the diet could result from the fact that their main Microtus prey was probably super-abundant and easily available around nesting sites of these owls. It is well documented that under such condition, where one prey species become super-abundant different owl species prey at the same site mostly on this super-abundat prey species (Mi k kol a 1983, Tay l or 1994, Houston et al. 1998). It is invisigated that in years when the main prey species is much less numerous, the degree of food niche overlap is much lower. The sample size was, however, not representative enough to give space for further generalisations on feeding relationships between the two owl species. ACKNOWLEDGMENTS: I greatly appreciate help of the following persons in collecting pellts: Patricia Kennedy and Bruce Park form the East Oregon Agricultural Research Station of the Oregon State University. Dr. Eric Forsman has confirmed identification of some prey items. 181 REFERENCES B egon M., Har p er J.L., Tow ns end C. R . 1990 – Ecology – Individuals, populations and communities – Blackwell Scientific Publ., London, UK, 2nd edition. Chapter 4. Her rera C.M., Hira ldo F. 1976 – Food-niche and trophic relationships among European owls – Ornis scand. 7: 29 –41. Houston C.S., Smit h D.G., R ohn C. 1998 – Great Horned Owl (Bubo virginianus) – Birds of North America, 372 – Philadelphia, Academy of Natural Sciences. Johnsgard P.A. 2002 – North American owls: biology and natural history, 2nd ed. – Washington, Smithonian Institution Press. Konig C., Weick R . 2008 – Owls of the world – London, Christopher Helm. Kopij G. 2006 – The structure of assemblages and dietary relationships in birds in south african grasslands – Wrocław, Wydawnictwo Akademii Rolniczej we Wrocławiu. Mar t i 1974 – Feeding ecology of four sympatric owls – Condor, 76: 45 –61. Mas er C., Stor m R .M. 1970 – A key to Microtinae of the Pacific North-West – O.S.U. Book Stores Inc., Corvallis, OR. Mi k kol a H. 1983 – Owls of Europe – Poyser, Calton. Rudolph S.G. 1978 – Predation ecology of coexisting Great Horned and Barn owls – Wilson Bul. 90: 134 –137. Shannon C.E., We aver W. 1949 – The mathematical theory of communication – Univ. Illinois Press, Urbana. Ty lor I. 1994 – Barn Owls: predator-prey relationships and conservation – Cambridge (UK), Cambridge University Press. Ya lden D.W. 2009 – The analysis of owl pellets – Carronades (U.K.) – The Mammal Society. Ver ts B.J., C ar raway L.N. 1998 – Land mammals of Oregon – University of California Press, Berkeley. Wo o dward G., Hi ldre w A.G. 2002 – Bodysize determinants of niche overlap and intraguild predation within a complex food web – J. Animal Ecol. 71: 1063–1074 Received after revision August 2012 journal 33.indb 181 2013-04-30 10:05:52