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Australian Mammalogy, 2009, 31, 75–80

Using the diet of the barn owl (Tyto alba) as an indicator of small vertebrate abundance in the Channel Country, south-western Queensland Matthew C. McDowell A,B,C and Graham C. Medlin A A

Mammal Section, South Australian Museum, North Terrace, Adelaide, SA 5000, Australia. School of Biological Science, The Flinders University of South Australia, PO Box 2100, Adelaide, SA 5001, Australia. C Corresponding author. Email: matthew.mcdowell@flinders.edu.au B

Abstract. The diet of the barn owl (Tyto alba) was determined by analysing pellets and bulk pellet debris found in the ruins of Baryulah Homestead, south-western Queensland. Nine species of mammal, at least eight bird, five reptile and three frog species were identified. The majority of prey consisted of small mammals and was dominated by Mus musculus, which accounted for almost 40 Prey Unit percent (PU%) of all prey. Rattus villosissimus was an important secondary prey species, which, due to its comparatively large mass, contributed 21.79 PU%. Other native mammals were present in low frequency only. Reptiles (primarily geckos) were more abundant than expected, collectively contributing >15 PU%, suggesting that they were an important component of the barn owl’s diet.

Introduction The Australian arid lands are amongst the most impacted of Australian ecosystems (Letnic 2000) and have experienced the highest rate of mammal extinctions anywhere in the world within the last 200 years (Morton 1990). Extensive grazing by introduced feral and domestic livestock, along with pressure from introduced predators, have been the primary threats to Australia’s biodiversity (Burbidge and McKenzie 1989; Morton 1990, 1995; Pickup and Stafford Smith 1993). Avenant (2005) found owl pellet analysis to be a useful tool for monitoring small mammal populations. Owls typically ingest their prey whole and later egest a mucous-covered pellet containing indigestible material such as bones and fur. Pellets are usually regurgitated at a roost in a cave, rocky overhang, tree-hollow or building, where they are protected from the elements and can be preserved for many years. The diet of an owl can be examined by identifying the contents of pellets, providing a biased sample of the small vertebrate fauna of the surrounding area. Owl pellet studies should not be considered a replacement for conventional biological survey techniques, but rather should be thought of as complementary to them (Torre et al. 2004; Avenant 2005). They can be especially useful when applied in remote regions where a small investment of time to opportunistically collect an owl pellet accumulation can yield a large quantity of data for an area that would otherwise be expensive or difficult to survey. The Channel Country Bioregion of south-western Queensland is inhabited by barn owls (Tyto alba), barking owls (Ninox connivens) and southern boobooks (N. novaeseelandiae) (McFarland 1992; Higgins 1999). The owl pellets used in this study were identified as barn owl pellets on the basis of their Australian Mammal Society 2009

characteristic size, shape, black to brown colour and glazed appearance (Andrews 1990; Hollands 1991). Barn owls prey predominantly on rodents and other small nocturnal terrestrial mammals, but also eat juvenile larger mammals, frogs, lizards, birds and insects (Morton et al. 1977; Morton and Martin 1979; Andrews 1990). The aim of this study was to use the barn owl pellet and bulk pellet debris accumulations found at Baryulah Homestead to investigate the composition of the surrounding small vertebrate fauna. Materials and methods Study site The Channel Country Bioregion of south-western Queensland is characterised by a series of anastomosing streams that meander southward across gibber floodplains and around sand dunes and undulating downs, turning west in the study area to flow towards Lake Eyre (Mabbutt 1988; McFarland 1992; Gibling et al. 1998). This network of channels has had a profound effect on the geomorphology and biology of the region. Summer floodwaters can fill the stream channels, and then inundate the surrounding plains with a shallow sheet of water up to tens of kilometres across. These rare flood events can take place regardless of local rainfall and can stimulate the flora, and consequently the fauna, to flourish in boom–bust cycles (Letnic and Dickman 2006). Cooper Creek remains dry most of the time but harbours several quasipermanent waterholes along its length, one of which occurs at Baryulah (Knighton and Nanson 1994). Waterholes support vegetation that provides refugia for native animals during drought, but are also important to pastoralists as stock-watering 10.1071/AM08116

0310-0049/09/020075

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Australian Mammalogy

points (Morton 1990; Pickup and Stafford Smith 1993; Morton et al. 1995; Letnic 2000). The south-western section of the Channel Country Bioregion is underlain by part of the largest and most prolific land-based hydrocarbon province in Australia. Santos Ltd has held ‘Authority to Prospect’ in south-western Queensland since the 1950s and has established numerous gas and oil wells in the region (Bennett 1997). However, the primary land-use of the region is cattle grazing (Morton et al. 1995). Under favourable conditions the Cooper floodplain has been perceived as providing some of the world’s best cattle-fattening pasture (Capon 2003). In the past these rare wetter periods generated unrealistically high expectations of stock carrying capacity, which has triggered land degradation during drier periods (Reid and Gillen 1988; Gibling et al. 1998; Pickup 1998). The Cooper Creek channels support coolabah (Eucalyptus coolabah) and river red gum (E. camaldulensis) woodland with lignum (Muehlenbeckia florulenta) and Queensland bluebush (Chenopodium auricomum) shrubland. The floodplain supports a variety of acacias (including Acacia cambagei, A. salicina and A. stenophylla), bignonia emu bush (Eremophila bignoniiflora), and Senna spp. shrubland/sparse Mitchell grass grassland. Nearby dunes support lobed spinifex (Triodia basedowii) and sandhill canegrass (Zygochloa paradoxa) hummock grassland with mulga (Acacia aneura), dead finish (Acacia tetragonophylla), ironwood (Acacia excelsa), beefwood (Grevillea striata) and bloodwood (Corymbia terminalis) shrubland (McFarland 1992; Williams 2003; G. Carpenter, pers. comm. 2007). The Durham Downs weather station, located ~75 km northnorth-east of Baryulah (Fig. 1), receives a mean annual rainfall of 220 mm but experiences a mean annual evaporation of 3200 mm (BOM 2007). The highest average monthly rainfall occurs in February (34.3 mm) and the lowest in September (9.2 mm). The hottest weather occurs in January, which has a mean monthly maximum of 37.0C and mean monthly minimum of 23.6C. The coldest weather occurs in July, which has a mean monthly maximum of 19.2C and mean monthly minimum of 5.6C (BOM 2007). Collection and analysis On 14 December 2002 whilst contracted to conduct a biosurvey for Santos, G. Carpenter and D. Armstrong found an owl pellet accumulation in the ruins of Baryulah Homestead (27340 S, 141400 E) near the banks of Cooper Creek (Fig. 1). They collected 208 whole barn owl pellets and ~4 L of decomposed pellet debris from the floor of a small log and daub building that had retained its galvanised iron roof. Though the time and rate of accumulation is unknown, several the pellets had retained some of their mucous coat, suggesting that they were relatively recent (Morton et al. 1977; Morton and Martin 1979). Individual pellets were soaked in warm water to dissolve the outer mucous coat and soften the pellet, sieved to remove water and then air-dried on a labelled paper towel. When sufficiently dry, pellets were dissected using forceps to separate diagnostic elements from fur. The decomposed pellet debris, or bulk sample, was sieved to remove excess sediment then placed in a cardboard tray and swept from one end to the other with a brush to

M. C. McDowell and G. C. Medlin

Fig. 1. Location of the Baryulah owl roost. WS = weather station.

expose diagnostic bones, which were picked out with forceps and placed in labelled containers. Due to uncertainty regarding the age, completeness and biasing of the bulk pellet debris, the results from both pellet and bulk samples were combined. Diagnostic bones were identified using published descriptions and comparative material from the South Australian Museum mammal, bird and reptile collections. Whole and part skulls, dentaries and/or teeth (where applicable) were used to identify mammals, birds and reptiles, and ilia were used to identify frogs. All diagnostic specimens were identified to the lowest taxonomic level possible (usually species). Most reptile specimens could not be identified beyond Family. All specimens were deposited in the South Australian Museum Subfossil Collection (Registration Nos SF8026–8522). The minimum number of individuals (MNI) was determined by counting the number of the most common diagnostic element of each species in each pellet. The MNI of each taxon from each pellet was then summed to determine an aggregate MNI. A similar process was used to determine the MNI of each taxon found in the bulk sample. This method is thought to minimise estimation biases caused by aggregation (Lyman et al. 2003). To facilitate comparisons between species, MNIs were converted to Relative Abundance (Ri%) and Prey Units (PU%) (after Morton and Martin 1979). Ri% is an expression of the MNI of any given species as a proportion of the sample’s total MNI. PU% was determined by allocating M. musculus a Prey Unit of 1 (assuming an average weight of 10 g) and scaling all other species accordingly (Appendix 1). Rattus villosissimus has an average adult weight of 134 g (Watts 1995), but because most specimens recovered in this study were juvenile, the species was allocated a Prey Unit value of 5 rather than the adult value of 10 or more (after Morton and Martin 1979). Results In total, 2165 prey individuals were identified from the assemblage (Table 1). Eight native and one introduced species of mammal, at least five reptile, eight bird and three frog

Barn owl diet and small vertebrate abundance


Table 1. The minimum number of individuals (MNI), relative abundance (Ri%) and percentage of prey units (PU%) for prey recovered from the Baryulah owl roost Species Mammalia Ningaui ridei Planigale gilesi Sminthopsis crassicaudata Sminthopsis macroura Sminthopsis sp. indet. Leggadina forresti Mus musculus Notomys sp. indet. Pseudomys hermannsburgensis Rattus villosissimus Aves Artamus cinereus Cincloramphus mathewsi Lichenostomus plumulus Malurus leucopterus Melopsittacus undulatus Pomatostomus ruficeps Taeniopygia guttata Turnix velox Passeriformes indet. Reptilia Nephrurus sp. Gekkonidae indet. (small) Tiliqua multifasciata Scincidae indet. Agamidae indet. Amphibia Cyclorana platycephala Litoria sp. indet. Neobatrachus centralis Neobatrachus sp. indet. Total Minimum number of species

MNI

Ri%

PU%

2 45 51 17 40 24 949 1 125 105

0.09 2.08 2.36 0.79 1.85 1.11 43.85 0.05 5.78 4.85

0.04 0.93 3.18 1.41 2.49 1.49 39.39 0.12 5.19 21.79

1 1 17 6 8 1 9 4 2

0.05 0.05 0.79 0.28 0.37 0.05 0.42 0.18 0.09

0.12 0.12 1.41 0.25 0.66 0.21 0.37 0.50 0.17

94 512 1 23 4

4.30 23.66 0.05 1.06 0.18

3.86 10.63 0.12 0.48 0.08

18 7 70 28

0.83 0.32 3.23 1.29

0.75 0.15 2.91 1.16

2165 25

100

100

species were identified. Small mammals made up most of the prey (76.05 PU%) but native small mammals were poorly represented compared with M. musculus, which contributed almost 40 PU%. R. villosissimus and P. hermannsburgensis were important secondary prey that contributed approximately equally to the samples’ MNI. However, due to its much greater mass R. villosissimus contributed 21.79 PU% whereas P. hermannsburgensis contributed 5.19 PU%. Birds and amphibians made minor dietary contributions but, interestingly, reptiles, which are not usually considered important in the diet of the barn owl, contributed 15.17 PU%. Reptilian prey consisted primarily of large and small geckos, which combined contributed the second highest relative abundance (27.96 Ri%) and the third highest prey unit value (14.49 PU%). Discussion The comparatively high abundance of geckos in the Baryulah assemblage indicates that nocturnal reptiles were an important dietary component of the Baryulah barn owls. High proportions

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of reptilian prey in the diet of Australian barn owls have been reported only once before (McDowell and Medlin 2009). The high proportion of gecko prey recovered from the Baryulah assemblage suggests that barn owls throughout the arid zone may be more dependent on geckos as a supplementary food supply than previously understood. Morton et al. (1977) considered M. musculus to be rare in south-western Queensland. Other studies conducted in the Channel Country Bioregion also reported M. musculus as absent or uncommon in owl pellets (Valente 1981; Debus et al. 1999; Debus and Rose 2003). However, M. musculus dominated the Baryulah accumulation and probably the surrounding small mammal community. M. musculus appears to be able to occupy any vacant niche, and is particularly successful in colonising disturbed areas (Fox and Gullick 1989; Singleton 1995; Haering and Fox 1997). The species has very short gestation and weaning periods (19 and 18 days respectively, compared with L. forresti and P. hermannsburgensis: 35 and 31 days gestation and 28 and 30 days weaning respectively) and reaches sexual maturity at a young age (Yom-Tov 1985; Strahan 1995), which may allow the species to make use of emerging resources more quickly than native rodents. However, if present at a sufficiently high population density, native rodents are thought to be able to outcompete M. musculus (Reid and Gillen 1988; Fox and Gullick 1989; Haering and Fox 1997). The dominance of M. musculus in the Baryulah collection suggests a high degree of environmental disturbance that is probably a result of European land use and grazing practices rather than mining activities. The study area is reported to have been strongly affected by cattle grazing (Pickup and Stafford Smith 1993; Pickup 1998; James et al. 1999) and is very close to Cooper Creek Road, making it easily accessible to tourists. Owing to its importance as a watering point for cattle (and people), Baryulah Waterhole has probably experienced particularly high vegetation and soil degradation, producing conditions that promote colonisation by M. musculus, but simultaneously discourage native small mammals. Morton (1990) suggested that a key factor in the demise of native mammals in the arid zone was their dependence on refuge habitats (persistent patches of high-quality nutritious vegetation often surrounding waterholes) to which individuals could retreat and survive during drought. Sattler and Williams (1999) noted that grazing by feral animals such as rabbits, camels, pigs and goats had particular impact on waterholes. The introduction of exotic herbivores has resulted in degradation of waterhole refugia, causing a collapse of the established system of retreat and recolonisation that characterises the ecology of many arid-zone species. This has resulted in the range reduction of most and the extinction of some native small mammals, leaving numerous empty niches for M. musculus to exploit. The species richness of dasyurids and rodents recorded in this study were comparable (see Table 1) but their frequencies differed greatly. Collectively, dasyurids contributed 8.05 PU% of the assemblage and were therefore thought to be less accessible as prey. Dasyurids are usually solitary, non-irruptive seasonal breeders and consequently form a smaller proportion of the prey base available to an owl (Heywood and Pavey 2002). Therefore, the low relative abundance of dasyurids in the diet of the barn owl at Baryulah probably accurately reflects their relative

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abundance in the surrounding community (Heywood and Pavey 2002). The barn owl(s) that lived at Baryulah appeared to prey primarily on rodents but also seemed to rely on reptilian prey, perhaps to cope with an unpredictable mammalian prey source. In contrast with previous studies of owl pellets in the Channel Country (Morton et al. 1977; Valente 1981; Debus et al. 1999; Debus and Rose 2003), small mammal prey consisted predominantly of M. musculus, suggesting that the study area has experienced high levels of environmental disturbance. With further research, the type and proportions of prey consumed by barn owls may become a useful measure of the health of the surrounding prey community and the environment on which they rely.

Acknowledgements This research was funded by Santos Ltd. We are very grateful to Steve Riley for his role in securing funding, and to Graham Carpenter and David Armstrong for collecting the owl pellet accumulation on which this research is based. We thank Catherine Kemper, David Stemmer, Mark Hutchinson, Philippa Horton and Maya Penck for access to their respective collections and aid in identifying specimens. We are grateful to Graham Carpenter, Catherine Kemper and Gavin Prideaux for providing comments on draft manuscripts. We also thank the Bureau of Meteorology for providing the relevant rainfall and flood data. Finally, thanks go to the numerous volunteers – particularly Brian Ross, Zbigniew Rudnicki and Janine Ellis – for their help with pellet dissection and sorting of bulk pellet debris. The quality of the paper was further enhanced by the constructive comments of three anonymous referees.

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Manuscript received 28 November 2008, accepted 6 May 2009

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Appendix 1. Prey Unit values for taxa from the Baryulah accumulation Mammal weights are after Strahan (1995). Bird weights are after Higgins (1999); Marchant et al. (1994); Higgins et al. (2001); Higgins and Peter (2003) and Higgins et al. (2006). As reptile and amphibian weights are rarely recorded they were estimated according to body size. Numbers in parentheses are averages Species Mammalia Ningaui ridei Planigale gilesi Sminthopsis crassicaudata Sminthopsis macroura Leggadina forresti Mus musculus Notomys sp. indet. Pseudomys hermannsburgensis Rattus villosissimusA Aves Artamus cinereus Cincloramphus mathewsi Lichenostomus plumulus Malurus leucopterus Melopsittacus undulatus Pomatostomus ruficeps Taeniopygia guttata Turnix velox Reptilia Nephrurus sp. indet. Gekkonidae indet. – small Tiliqua multifasciataB Scincidae Agamidae Amphibia Cyclorana platycephala Litoria sp. indet. Neobatrachus centralis

Mass (g)

Prey units

6.5–10.5 5–16 10–20 (15) 15–25 (20) 15–20 8–25 (10) 30–50 (35) 9–14.5 (12) (134)

0.75 0.5 1.5 2 1.5 1 3 1 5

(35.3) (30.25) (19.2) (8.5) (26.3) (50. 5) (12.4) (45.0)

3 3 2 1 2 5 1 3

~10 ~5 ~20 ~5 ~5

1 0.5 2 0.5 0.5

~21 ~5 ~10

2 0.5 1

A

The majority of R. villosissimus specimens were juvenile and have been attributed fewer prey units than would be appropriate for an average adult. B The individual recovered was a juvenile so weight was estimated accordingly.

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