Polish Journal of Ecology Pol. J. Ecol. (2014) 62: 801–805
Short research contributions
DIETARY OVERLAP OF CO-OCCURRING BARN OWL TYTO ALBA SCOPOLI AND SPOTTED EAGLE OWL BUBO AFRICANUS TEMMINCK IN URBAN AND RURAL ENVIRONMENTS Grzegorz KOPIJ 1*, Craig T. SYMES 2, Robin BRUYNS 2 Department of Wildlife Management, University of Namibia, Private Bag 1096 Katima Mulilo, Winela Rd., Namibia, * e-mail:
[email protected] (corresponding author) 2 School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa 1
ABSTRACT: Diet of co-occurring Barn Owl and Spotted Eagle Owl has been studied by means of pellet contents analysis in urban and rural environments in the Highveld of South Africa. In urban environment, diet of both owl species was dominated by murid rodents (mainly Otomys, Mastomys and Rhabdomys). In rural environment, Barn Owl diet was also dominated by murid rodents, but in the diet of the Spotted Eagle Owl higher proportion of birds and non-murid rodents was recorded. Although in the rural environment the breadth of diet niche was wider in Spotted Eagle Owl (DB = 35.41) than in Barn Owl (DB = 12.67), there was almost total dietary overlap (DO = 0.98) between these two co-occurring owl species. For contrast, there was only slight food niche overlap (DO = 0.12) between these owl species co-occurring in the urban environment, but the diet breadth here was also wider in Spotted Eagle Owl (DB = 29.02) than in Barn Owl (DB = 17.90). In the urban environment diet breadth of the Spotted Eagle Owl is, therefore, slightly wider than in rural environment, while in the case of the Barn Owl the reverse is true. Probably there is lower abundance of available prey in urban and rural areas in the Highveld, in comparison with more natural habitats. This may force both species to resort to a more diverse diet to meet their energy requirements. Both species show, therefore, high plasticity of foraging.
Many studies have focused on the breeding and population dynamics of avifauna in urban environments with respect to their surrounding natural area (e.g., D e Graaf et al. 1991) but much less have focused on the diets of birds, particularly raptors, in an urban environment (e.g., Sy mes and Kr u ge r 2012). Urban environments are generally not considered fully functional systems due to the fact that natural predator/prey are often in decline or are absent altogether (Ol kow sk i et al. 1976). Such relationships may include those between owls and their prey, although in some urban areas certain owl species may have adapted to development due to the persistence of rodents (Me ye r 2008, C av i a et al. 2009). This persistence in transformed areas is probably mediated by their ability to undergo prey switching in response to the temporal and seasonal availability of prey (Ti ls on and L e R ou x 1983, Kopij 2006). In southern Africa, the Barn Owl Tyto alba Scopoli 1769 and the Spotted Eagle Owl Bubo africanus Temminck 1821 are the two most common and widespread owl species, which often co-occur in the same habitats (Ho cke y et al. 2005). Although their diet has been relatively well documented (Ho cke y et al. 2005, Kopij 1997, 2006), the food com-
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position of sympatrically occurring Barn Owl and Spotted Eagle Owl has been analysed hitherto in a few natural sites only, i.e. desert (Ti ls on and L e R ouix 1983), grassland (D e an 1973), savanna (D e an 1973, Ver non 1980, Mendels ohn 1989, Dixon and Perr in 1994) and forest (S chneider and Jorge 2007). Consequently, the aim of this study was to investigate the diet of these two owl species in human-modified environments. Because we were particularly interested in the co-occurrence of two avian nocturnal predators with similar diets, we hypothesized that selection for different prey items would result in the distinct niche partitioning of these avian predators. In the rural environment, Barn Owl’s pellets were collected in dry season from a cave at Avondzon near Fouriesburg, eastern Free State province, South Africa, in October 1996 (Table 1). The Spotted Eagle Owl’s pellets were collected in four sites: Avondzon in October 1996, Wolluterskop in October 1996, Sunnyside in August 1994, and Vaalbank in October 1996 (Table 1). Only fresh and compact pellets were collected. All these sites are located along sand-stone cliffs in the Cympopogon-Themeda Veld (Aco cks 1975) utilized as pasture for cattle, sheep, and goats. In the urban environment, pellets of both owl species were collected from eight sites in the Gauteng Province and from one site in the Free State province, during April to July
2009 (Table 1). Pellets were collected opportunistically in response to requests made to the public via e-mail and local newspapers. A company that builds and maintains owl boxes for the public was approached, with the idea that pellet collection would be made easier through identifying and locating active nest boxes. Off an estimated 300-350 nest boxes erected in Johannesburg and surrounding areas, only 2-3% (9 erected boxes) were possibly active (R. Bruyns, pers. obs.). The degree of urbanization ca 1 km around the nesting site (average territory size) ranged from holdings lying near the centre of the city or town, to sites at the urban-rural border. Pellets were analysed using universally accepted method (cf. Kopij 1997, 2004). In laboratory, each pellet was broken apart in hand and the remains of the prey items were identified using reference collections for small mammals, birds and insects at the Ditsong National Museum of Natural History (formerly Transvaal Museum). Skulls and dentures of mammals, skulls, feet and feathers of birds, as well as heads, mandibles, and legs and elytra of insects were used for this identification. Dietary Breadth (DB) and Dietary Overlap (DO) were calculated according to Panka (1976): DB = 1/Σp2 x (l/n) (1) where: p is the proportion of given prey taxon, n is the number of prey taxa
Table 1. Collection sites for owl pellets. Environment Site’s name Spotted Eagle Owl Bubo africanus: De Wilgers Bergbron Benoni Urban Beverly Sandton Petervale Rural Avondzon Wolluterskop Vaalbank Sunny Side Barn Owl Tyto alba: President Park Urban Glenferness Denysville Rural Avondzon
Province
Co-ordinates
Collection date
No.pellets
Gauteng Gauteng Gauteng Gauteng Gauteng Gauteng Free State Free State Free State Free State
25°45′48″S-28°18′40″E 26°10′12″S-27°57′34″E 26°06′23″S-28°18′07″E 26°00′30″S-28°01′04″E 26°03′17″S-28°03′13″E 26°02′57″S-28°02′41″E 28°32′08″S-28°31′11″E 28°14′00″S-28°18′05″E 28°15′00″S-28°32′07″E 28°32′08″S-28°31′11″E
April-July 2009 April-July 2009 April-July 2009 April-July 2009 April-July 2009 April-July 2009 October 1996 October 1996 October 1996 August 1994
11 12 6 1 3 5 6 2 27 10
Gauteng Gauteng Free State Free State
26°00′45″S-28°08′26″E 25°59′19″S-28°02′19″E 26°53′27″S-28°06′01″E 28°32′08″S-28°31′11″E
April-July 2009 April-July 2009 April-July 2009 October 1996
7 60 4 80
Diet of Barn Owl and Spotted Eagle Owl
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Table 2. Diet composition of Tyto alba and Bubo africanus in urban and rural environment in South Africa. %N – percentage of prey items; %W – percentage of biomass. In brackets subtotal values for orders/classes are given. Prey taxa
Chiroptera Scotophilus dingani Chiroptera spp. Insectivora Myosorex varius Crocidura hirta Crocidura fuscomurina Crocidura cyanea Crocidura spp. Rodentia Otomys sp. Rhabdomys pumilio Mastomys sp. Mastomys/Rhabdomys/Mus Murinae spp. Cryptomys hottentotus Xerus inauris Rodentia spp. Aves Passer sp. Ploceus velatus Small (sparrow-size) Medium (dove-size) Large (chicken-size) Unidentifiable Insecta Orthoptera spp. Ort.: Gryllacrididae sp. Mantodea sp. Col.: Scarabidae spp. Number of prey items Total biomass (g)
Urban environment T. alba B. africanus %N %W %N %W (0.0) (0.0) (2.6) (4.3) 2.6 4.3 (4.4) 2.6
(0.8) 0.6
0.9 0.9
0.1 0.1
(82.8) 9.5 6.0 7.8 59.5
(94.5) 19.8 4.7 9.0 59.5
0.9
(8.6) 5.2 3.4
(3.4) 3.4
(20.5)
(9.6)
17.9 2.6
9.3 0.3 0.0
(61.6) 10.3
(84.1) 24.8
51.3
59.3
Rural environment T. alba B. africanus %N %W %N %W (3.8) (1.6) (13.3) (2.4) 3.8 (0.0)
1.6 (0.0)
13.3 (2.2)
2.4 (2.2)
(87.6) 6.3
(89.1) 15.4
2.2 (68.9) 46.7
0.2 (61.7) 49.8
20.0
11.9
2.2
12.7
(11.1)
(23.0)
2.2
0.7
6.7 2.2
12.2 10.1
(4.4)
(0.1)
2.2 2.2
0.0 0.0
66.3
56.0
15.0 (8.8)
17.7 (9.3)
7.5 1.3
4.0 5.3
(0.0)
(0.0)
1.5
(4.5) 2.3 2.2
(0.2) 0.2
(2.6)
2.6 (12.8)
12.8 39
116 6490
DO = Σ(p1 x p2)/ √(Σp12 x Σp22) (2) where: p1 and p2 are proportions of a given prey taxon in the diet of the Barn Owl and Spotted Eagle Owl, respectively. These values range from 0 (no overlap) to 1 (complete overlap). DO was calculated on the coarse resolution, using prey orders in the case of mammals and classes in the case of other prey taxa. The weight of mammals is taken from Sk inner and Smit hers (1990), and birds from Ho cke y et al. (2005).
(1.6)
1.6 (0.5)
0.5 80 1890
45 3787
4935
In the urban environment, the diet of the Barn Owl (both in terms of the number of prey items and their biomass) consisted primarily of rodents, with murids as the most important group (Table 2). The diet was supplemented by shrews and birds of sparrow size and at least 11 prey species were recorded in total (Table 2). In the same area the Spotted Eagle Owl diet was also dominated by rodents (both numerically and by weight), and the main group within this order were also murids, with predominating Otomys and
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Grzegorz Kopij et al.
Mastomys species (Table 2). Shrews, birds, bats, and scarabaeid beetles supplemented the diet and seven prey species were recorded in total (Table 2). In the rural environment, the items in Barn Owl diet (both numerically and by weight) comprised murid rodents (at least two species), with bats and birds as supplementary food (at least three species). In the diet of the Spotted Eagle Owl at least 10 prey species were identified in this habitat. In comparison with Barn Owl diet, higher proportion of birds and non-murid rodents were recorded (Table 2). Although in the rural environment the diet variation was wider in Spotted Eagle Owl (DB = 35.41) than in Barn Owl (DB = 12.67), there was almost total dietary overlap (DO = 0.98) between these two co-occurring owl species. For the contrast, there was only slight food niche overlap (DO = 0.12) between these owl species co-occurring in the urban environment, but the diet breadth here was also higher in Spotted Eagle Owl (DB = 29.02) than in Barn Owl (DB = 17.90). In the urban environment, the diet breadth of the Spotted Eagle Owl is, therefore, slightly higher than in rural environment, while in the case of the Barn Owl the reverse was true. The wider diet breadth in Spotted Eagle Owl in comparison with the Barn Owl may partly be attributed to larger sample size in the former species (Table 1). Small mammals, and among them mainly rodents, constitute invariable main food of both the Barn Owl and Spotted Eagle Owl throughout the Ethiopian Zoogeographical Region (Ho cke y et al. 2005). All other main prey groups occur in the diet with a frequency ranging from 16.6% to 83.3% (Ver non 1980, Ho cke y et al. 2005). In overall, arthropods comprised 2/3 of all prey items in the diet of the Spotted Eagle Owl in southern Africa, although they were not recorded in all studies (Ver non 1980, Ho cke y et al. 2005), In this study, they were an insignificant component, but pellets were collected mainly in dry season, when insects are generally scarce in the grassland. At most southern African sites, rodents, shrews and birds accounted for more than 96% of vertebrate prey, with rodents comprising 75-97% (D e an 1973, Kopij 2004, Ho cke y et al. 2005), which was confirmed by this study.
Owls in the genus Bubo have been described as generalist, irrespective of the area they breed (R ieger t et al. 2007, Kopij 2013). This study showed the Spotted Eagle Owl diet in urban areas consisted of rodents, shrews, bats, and insects. The ability for Spotted Eagle Owls to hunt both on the ground and on the wing with equal effect (Ho cke y et al. 2005) probably allows them to benefit when certain prey species numbers are low. In highly modified urbanized areas, prey species numbers may be low due to habitat degradation and corresponding low biodiversity, yet this still allows Spotted Eagle Owls to persist by having the ability to hunt a wider range of prey items. Therefore, by having a broader diet, Spotted Eagle Owls, as generalists, are able to buffer the effects of low abundances of certain prey with that of other prey items. Ave nant (2007) sampled 400 pellets from a roost 4.5 km from Soetdoring Nature Reserve near Bloemfontein, South Africa. Even though this site is ca 300 km from any of sample sites mentioned in this study, the small mammal prey species identified from the pellets are similar to those found in this study (R. pumilio, Mastomys sp., Otomys sp., M. varius, C. cyanea and C. hottentotus). This example highlights each genus as potential preferred prey for the Barn Owl on the Highveld of Southern Africa. The fact that these prey species are found in natural, rural, periurban and some in urban areas, also highlights the prey species ability to exploit new environments. These allow owl populations to persist as long as the prey population is present (B e r r y et al. 1998). Bird remains recovered from Glenferness belong to the genus Ploceus. The occurrence of weavers in only a sub-set of pellets spanning 4-5 days implies an opportunistic feeding event. Weavers build individual nests in a small community within the same tree. So a Barn Owl finding such a tree would be able to hunt birds repetitively over a few days until the birds either moved away from the area or the numbers become too few (own observation). Bird remains found in pellets from President Park belong to the sparrows of the genus Passer, most likely taken while flying at dusk or while the birds were roosting. Another instance of opportunism is evident with the presence of Orthoptera specimens.
Diet of Barn Owl and Spotted Eagle Owl
Both species of owls showed similar diet diversity compared with other studies (Ho cke y et al. 2005). From the data gathered, it is evident that there is a low abundance of available prey in urban and rural areas, forcing both species to resort to a more diverse diet to meet their energy requirements. The ability to diversify the use of a broad prey base enables them to be successful predatory species across a wide distribution range. Similar high plasticity in foraging was shown for urban population of the Tawny Owl (Gr zę dzick a et al. 2013). ACKNOWLEDGMENTS: Teresa Kearney, Ruth Müller and Tamar Cassidy from the Ditsong National Museum of Natural History (formerly Transvaal Museum), Pretoria (Northern Flagship Institution) are thanked for assistance in identification of small mammals, insects and birds respectively. Members of the public are thanked for assistance in pellet collection. REFERENCES Aco cks J.P.H. 1975 - Veld types of South Africa - Mem. Bot. Surv. S.A. 40: 1-128. Avenant N.L. 2007 - Barn Owl pellets: a useful tool for monitoring small mammal communities? - Bel. J. Zool. 135: 39-43. B er r y M.E., B o ck C.E., Haire S.L. 1998 - Abundance of diurnal raptors on open space grasslands in an urbanized landscape - Condor, 100: 601-608. C av i a R ., Cueto a G.R ., Su áre z O.V. 2009 Changes in rodent communities according to the landscape structure in an urban ecosystem – Landsc. Urban Plan. 90: 11-19. D e an W.R .J. 1973 - The ecology of owls at Barberspan, Transvaal (In: Proc. Symp. Afr. Pred. Birds, Ed: A.C. Kemp) - Pretoria, Northern Transvaal Orn. Soc. pp. 25-45. D e Graaf R .M., G eis A.D., He a ly P.A. 1991 - Bird population and habitat surveys in urban areas – Landsc. Urban Plan. 21: 181-188. Dixon J.E.W., Per r in M.R . 1994 - Prey selection by owls at Etosha, Namibia - J. Orn. 135: 172. Grzę dzicka E., Kus K., Nabiele c J. 2013 - The effect urbanizationon the diet composition of the Tawny Owl (Strix aluco L.) - Pol. J. Ecol. 61: 391-400. Ho cke y P.A.R ., D e an W.R .J., Ryan P.G., Mare e S. 2005 - Roberts’ birds of southern Africa. 7th ed. - John Voelcker Bird Book Fund, Cape Town, 1296 pp.
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Received after revision September 2014
