Feeding Ecology of the Barn Owl, Tyto alba, in ... - CSIRO Publishing

Aust. Wildl. Res., 1979, 6 , 191-204

Feeding Ecology of the Barn Owl, Tyto alba, in Arid Southern Australia

S. R. Morton and A. A. Martin Department of Zoology, University of Melbourne, Parkville, Vic. 3052.

Abstract

In arid parts of Australia the barn owl appears to feed largely on rodents which form irruptions or plagues, i.e, undergo marked changes in abundance. Barn owls became common at the height of an irruption of house mice, Mus musculus, in western New South Wales, but were comparatively scarce after the mice decreased in numbers. There was some evidence that the owls' diet, determined by analysis of pellets, was more varied immediately after the numbers of mice decreased, but its major part still consisted of M , musculus. The mean number of prey units represented in each pellet rose during the irruption and then declined to the original level. At a variety of sites in arid New South Wales and South Australia, barn owls' diet consisted almost entirely of small mammals. The most common prey species were rodents that fluctuate widely in abundance, and the mean amount of prey per pellet differed greatly among the study sites. The feeding ecology of barn owls in arid Australian environments is essentially similar to that described for more mesic habitats; hence, a greatly increased variation in the abundance of mammalian prey has not led to an increase in breadth of food niche.

Introduction The barn owl, Tyto alba (Scopoli), occurs in a wide variety of habitats in all continents, and is represented in Australia by T. alba delicatula (Gould). This race is morphologically uniform throughout the continent and thus is probably a recent immigrant (Mayr 1944; Mees 1964). Its range includes the semiarid and arid regions of Australia, where it frequently becomes locally abundant. A previous study (Morton et al. 1977) showed that the Australian barn owl was dependent upon prey species, such as the plague rat, Rattus villasissimus, that fluctuate from rarity to great abundance (Newsome and Corbett 1975); consequently, the owls are apparently exposed to the problems of a locally unstable food resource. The present study was designed to explore further the degree to which the diet of barn owls of the Australian arid zone is restricted to such rodents, and to investigate the possibility that the owls may overcome rapid reductions in prey numbers by switching to other food items. The diet of the barn owl was studied by analysis of regurgitated pellets. These pellets are particularly amenable to analysis because the cranial remains that they contain accurately reflect the number and identity of the prey consumed (Wallace 1948; Glue 1974; Raczynski and Ruprecht 1974). This study consists of two parts: first, the diet of barn owls in arid western New South Wales was examined throughout an irruption of introduced house mice, Mus musculus; and second, the diet of owls was determined over a broad area of arid New South Wales and South Australia.

S. R. Morton and A. A. Martin

Study Sites

Detailed studies of the diet of barn owls in relation to the abundance of their small mammal prey were carried out at Fowlers Gap Station, N.S.W.; samples of pellets were collected to determine the diet of barn owls at 10 other localities in arid western New South Wales and northern South Australia. Fowlers Gap Station Fowlers Gap Station (31°05'S., 141°45' E.) is 110 km north of Broken Hill, N.S.W. (Fig. 1). To the west of Fowlers Gap Homestead are rocky hills, but these fall away in the east to flat alluvial plains. Creeks, which are usually dry, run from the hills to join Fowlers Creek; the latter traverses the alluvial plains to the north-east. Both the hills and the plains carry low shrubland of Atriplex spp., Bassia spp. and Kochia spp., or grassland, Astrebla spp.; vegetation is usually sparse and less than 70 cm high. Areas of denser vegetation occur along creek banks and smaller drainage channels. The average annual rainfall is 195 mm. Barn owls roosted in river gums, Eucalyptus camaldulensis, that were confined to creek beds; roosts were found by walking along Fowlers, Homestead, Picnic and Rocky Creeks within 5 km of the Homestead and searching for pellets scattered beneath the trees. Samples of pellets were collected at irregular intervals (usually 2-4 months) between October 1974 and June 1976. Analysis of the pellets is described below. Abundance of small mammals was measured in two ways. First, live trapping was carried out with folding aluminium traps or, occasionally, wire-cage traps at irregular intervals between August 1973 and May 1976. Traps were set in dense vegetation along drainage channels and creek banks, and were baited with a mixture of rolled oats and peanut butter. The number of trap-nights per visit varied from 60 to 200. Second, spotlighting was used to sample the small mammals inhabiting the alluvial plains; Morton (1978) provided details of this method. Spotlighting was carried out at 2- to 4-monthly intervals from May 1973 until May 1976, and the number of spotlighting-hours per visit varied from 7.7 to 33.4. Six species of small animals were found at Fowlers Gap. Four were dasyurid marsupials: Planigale tenuirostris and Sminthopsis crassicaudata inhabited the open alluvial plains; P, gilesi and S. macroura probably occurred mainly in denser vegetation along the margins of creeks. The two remaining species were murid rodents: the introduced house mouse, Mus musculus, occurred at varying densities in all habitats; Leggadina forresti, a native species, was found sporadically on the open alluvial plains. Other Study Sites Pellets were collected from beneath owl roosts at 10 other localities (Fig. 1). Unless otherwise stated, all samples were collected in September 1975. Algebuckinna. Pellets were collected from the ruins of the old stone railway building at Algebuckinna (27"56' s., 135" 49' E.), 60 km south-east of Oodnadatta, S.A. No owls were seen. The habitat around the site was stony, rolling country with scattered bushes of Atriglex, Kochia and Acacia, but 1 km to the north lay the better vegetated floodplain of the Neales River.

193

Diet of Barn Owl in Arid Australia

Coober Pedy. Two roosts were found in coolibah trees, Eucalyptus microtheca, in 16-Mile Creek (at 29'14' S., 134'47' E.), 25 km south of Coober Pedy, S.A. No owls were seen. Around the creek were flat plains carrying primarily Atriplex spp. and Bassia spp.

0 Kenmore

Park

/

Lagoon

I

.I

Qld.

hanite Downs. Oodnadatta

Coober Pedy

S.A.

Fig. 1. Map of part of southern Australia showing the study sites (underlined) and other localities mentioned in text. Positions of the 250-mm and 125-mm annual average rainfall isohyets are also shown. The inset shows the area of Australia covered by the main map.

Erudina. Pellets were found beneath the hollow spout of a broken-limbed river gum in Wilpena Creek, 2 km west of Erudina Homestead, S.A. (31'29' S., 139'23' E.).

194


No owls were seen. A zone of sandy ground scattered with Nitraria schoberi surrounded the creek, and graded into flat plain covered in low Atriplex spp. and Kochia spp. Etadunna. Owls were found roosting in the coolibah trees lining Lake Killamperpunna (28'35' S., 138'44' E.), 20 km north-west of Etadunna Homestead, S.A. Five roosts were found, together with an abandoned nest and an apparently occupied nest in hollows; pellets from two of the roosts were collected. The band of trees surrounding the lake was about 0.1 km wide, and beyond lay the Natteranie sandhills. Vegetation, mostly ephemeral, was confined almost entirely to the bases of the sandhills. Granite Downs. Three pellet accumulations were found in Indulkana Creek at 26'58'S., 133'33' E., 12 km east of Granite Downs Homestead, S.A. One was beneath a river gum, one beneath an unidentified species of eucalypt, and the third beneath a mulga shrub, Acacia aneura; no owls were seen. Sandy flats surrounding the creek carried low vegetation of Atriplex spp. and ephemeral forbs, and these gave way to stony flats and rises, with scattered mulga about 1 km away on each side. Kenmore Park. Two roosts were found in river gums in the bed of Number 6 Creek (at 26'09' S., 132'49' E.), 35 km north of Kenmore Park Homestead, S.A.; no owls were seen. The surrounding areas were sandy plains and rocky outcrops covered in sparse, low grass and carrying scattered mulga and corkwood, Hakea sp. Lake Harry. Pellets were found in the ruins of Lake Harry Homestead (29'26' S., 138'15' E.), 30 km north-east of Marree, S.A. No owls were seen. The rolling stony plain surrounding the site carried a low growth of Atriplex spp., Bassia spp, and many ephemerals. Mount King. Two roosts were found in coolibah trees in Mokely Creek, 1 km west of Mount King Homestead (2ga14' S., 141°54' E.), 26 km north-west of Tibooburra, N.S.W., in February 1975. Owls were present at both roosts. The floodplain around the creek supported sparse Bassia spp. and Atriplex spp., and about 0.5 km on either side were rolling stony plains covered with similar vegetation. Information on the small mammals of this area was provided by Denny (1975). Warrina. Pellets were collected from the ruins of the old stone railway building at Warrina (28"12'S., 135'50' E.), 85 km south-east of Oodnadatta, S.A. No owls were seen. Habitat around the site was very similar to that at Algebuckinna, but 0.5 km to the south lay more heavily vegetated land around Bungadillina Creek, and 0 . 5 km to the north were sandy ridges carrying sparse Acacia burkittii. William Creek. Two roosts were located in William Creek 2 km west of William Creek township, S.A. (28'56' S., 136'20' E.). Both groups of pellets were beneath coolibah trees; no owls were seen. Sandy country with Atriplex spp., grasses and scattered Acacia aneura and A. burkittii surrounded the site. Only complete pellets were collected from the roosts. Observations of extensive pellet accumulations at Fowlers Gap and Mount King suggested that, in this arid environment, the surface coating of mucus on the pellets took about 3 months to break down. Few pellets collected in this study had lost this coating, and it is probable that most were collected within 3 months of deposition; it is unlikely that any were more than 6 months old. Owls were not present at all of the above roosts, but it is assumed that the pellets were those of the barn owl. Since the barn owl was the only species of Tyto known to be abundant in inland Australia such an assumption seemed reasonable, but during


1975 and 1976 the grass owl, T. capensis, was reported from several localities around Goyder Lagoon in north-eastern South Australia (Cox 1976; Parker 1977). The closest of our study sites to Goyder Lagoon is Etadunna (see Fig. 1); six owls were observed at close range at this site, and all were barn owls. Further, grass owls are slightly larger than barn owls and probably eject larger pellets (Vernon 1972; see also Dwyer and Willmer 1975). The largest pellets analysed in the present study were those from Etadunna, and were known to be those of the barn owl. It seems unlikely, therefore, that pellets of the grass owl are included in this study.

Method of Analysis Pellets were weighed and the remains of prey in each were identified and counted. The number of mammals in each pellet was determined from the number of lower jaws found, and species were identified by cranial characteristics (information on rodents was supplied by J. A. Mahoney and on dasyurid marsupials by M. Archer). Specimens of all native species of mammals found in the pellets at each site have been deposited in either the South Australian Museum, Adelaide, or the Australian Museum, Sydney. The number of lizards in each pellet was determined from the number of lower jaws found, the number of birds from crania, and the number of frogs from ilia. To facilitate comparisons between the diets of barn owls at the different sites, each vertebrate prey item was assigned a number of 'prey units', one prey unit being equivalent to a small mammal weighing 15-20 g (Southern 1954; Glue 1974). These conversion factors are: Leggadina for resti Notomys alexis Pseudomys hermannsburgensis Rattus villosissimus Mus musculus Unidentifiable murid Oryctolagus cuniculus (juv.) Chiroptera

1.5 2.0 1 .0 6.0 1 .O 1.0 4.0 1.0

Planigale gilesi Planigale tenuirostris Sminthopsis crassicaudata Sminthopsis macroura Sminthopsis ooldea Aves Lacertilia Anura

1.0 0.5 1.0 1 .0 1.0 2.0 0.5 0.5

Morton et al. (1977) scored the plague rat, Rattus villosissimus, as 5 prey units rather than the adult value of 7-8, on the basis that most rats captured by barn owls in that study were juvenile. In the present study, more detailed analysis showed that the plague rats captured by barn owls includgd many adult individuals, and hence a larger conversion factor (6) has been used. The ages of most specimens of R,villosissimus were assessed by the tooth-wear critera developed for the Australian bush rat, R . fuscipes, by Warneke (1971). The classes were: 1, 4.5 weeks; 2, 6 . 0 weeks; 3, 9.5 weeks; 4, 14.0 weeks; 5, 23.0 weeks; 6, 33.0 weeks; 7, 46.0 weeks; 8, 58.0 weeks. These age classes, since they are derived from laboratory study of a different species, obviously apply only very approximately to free-living R. t;illosissimus, but they provide some information on the age-range of individuals captured by barn owls.

Results Fowlers Gap Station Results of analysis of pellets are presented in Table 1. Nearly all the unidentifiable murid skulls were of juveniles, and lacked teeth, and it seems very likely that all were M. musculus. Only one of the birds found in the pellets could be identified (Cinclorhamplzus matlzewsi). The frogs included 22 Cyclorana platyceplzalus, one Limnodynastes tasmaniensis and eight unidentifiable specimens. Remains of 21 orthopterans and eight beetles were also found in the pellets. Changes in the abundance of small mammals at Fowlers Gap are shown in Fig. 2. Very few individuals of Sminthopsis macroura, Planigale gilesi and P. tenuirostris were captured, and these species are not included in the graph. There was a peak in the abundance of M. musculus and S.crassicaudata in late 1974 and early 1975; this

196


increase followed consistent vegetation growth caused by the unusually heavy rains throughout late 1973 and 1974 (see Morton 1978). The numbers of M. musculus trapped in creek beds and drainage gullies increased steadily during 1974, and in late Table 1. Prey items in barn owl pellets from Fowlers Gap Station Values are numbers and percentages of individuals for each date; the totals also include the percentages of prey units (PU)

%

N

%

N

Oct. Rodentia Leggadina forresti Mus musculus Unidentifiable murids Chiroptera Marsupialia Planigale gilesi P. tenuirostris Sminthopsis crassicaudata S, macroura Aves Anura

May 1975

-

-

-

-

96 8

278 5

-

219 6 -

95.7 2.6

-

1 1

1 2

0.4 0.9

1

0.4

-

-

25

26

56

-

-

June 1976

-

134 3 -

96.4 2.2

1

0.7

1

-

0.7 -

-

-

-

-

50

-

46

Nov. 1975 Rodentia L,forresti M. musculus Unidentifiable murids Chiroptera Marsupialia P,gilesi P. tenuirostris S. crassicaudata S . macroura Aves Anura

-

-

3

1

Number of pellets

%

N

Feb.

-

Number of pellets

%

N

-

32

53

15

--

N L, forresti M , musculus Unidentifiable murids Chiroptera

2 1311 46 3

% 0.1 91.3 3.2 0.2

Totals for 303 pellets PU 0.2 91.3 3.2 0.2

P, gilesi P. tenuirostris S. crassicaudata S,macroura Aves Anura

N

%

PU

15 1 8 4 15 31

1.0 0.1 0.6 0.3 1.0 2.2

1.0 0.1 0.6 0.3 2.0 1.1

1974 house mice became common on the plains surrounding the creeks; however, they did not appear to become resident in this habitat, and disappeared within 6 months. The increase in the numbers of M. musculus and S. crassicaudata in the


plains habitat was almost certainly much greater than is apparent from Fig. 2, because spotlighting became more difficult as vegetation increased in density. Hence, there were peak populations of M. musculus in both the creek bed and plains habitats during late 1974 and early 1975, and of S. crassicaudata in the plains habitat during

Fig. 2. Measures of the abundance of three species of small mammal at Fowlers Gap Station from 1973 to 1976. o Mus musculus (trapping estimate). 8 M. musculus (spotlighting estimate). Sminthopsis crassicaudata (spotlighting estimate), Leggadina forresti (spotlighting estimate).

Fig. 3. Percentage of prey units consisting of Mus musculus (o) and mean number of prey units per pellet (8) in barn owl pellets from Fowlers Gap Station, 1974-76.


the same period. Leggadina forvesti remained uncommon throughout the whole period of study. Barn owls fed primarily on M. musculus (Table l), and Fig. 3 shows the proportion of prey units comprised of M. musculus and the number of prey units per pellet during the study. In this graph, unidentifiable murids are included within M. musculus. The proportion of M. musculus in the diet dropped in May 1976; other prey items commonly eaten at this time were P. gilesi, birds and frogs (Table 1). More important, the mean number of prey units per pellet (and, therefore, the number of M. musculus per pellet) increased from 4.2 in October 1974 to 5.3 in December 1974; this increase coincided with the period of peak numbers of M. musculus (Fig. 2). The number of prey units per pellet then gradually fell to its original level over the next 12 months as prey numbers declined. Table 2. Prey items in barn owl pellets from study sites east of Lake Eyre Values are numbers and percentages of individuals, with percentage of prey units (PU)

Erudina Rodentia L. forresti - - - R. villosissimus M . musculus 137 97.2 97.9 Unidentifiable 1 0 . 7 0 . 7 Lagomorpha 0. cuniculus Marsupialia P. gilesi - - P. tenuirostris - S. cvassicaudata - S , macroura 1 0.7 0.7 Sminthopsis sp. - - - Aves Lacertilia 2A 1 . 4 0 . 7 - - Anura

Etadunna

5 5.1 4.0 17 17.2 54.1 74 74.7 39.3

-

No. of pellets A

Probably gekkonids.

31

Mount King

Lake Harry

-

-

-

-

-

-

-

-

-

-

-

-

1

1.0

0.5

2

2.0

2.1

129

48

246

Two agamids and four probable gekkonids.

It was not possible to estimate the density of barn owls during this study, but there is little doubt that their numbers increased in late 1974 to a peak in early 1975. In February 1975 many owls were heard calling along a 3- to 5-km stretch of Fowlers Creek; screeching is usually indicative of breeding (Bunn and Warburton 1977). At this time it was common to see owls hunting at night near Fowlers Creek, and roosts were numerous in all the creeks investigated. By August 1975 it was difficult to find roosts, and very few owls were seen.

Other Study Sites Results of analysis of pellets from study sites east of Lake Eyre are shown in Table 2 and from those west of Lake Eyre in Table 3. At all sites but one the most important prey item (in terms of prey units) was either M. musculus or R. villosissimus;


the exception was Coober Pedy, where L. forresti and Notomys alexis were the major prey species. As at Fowlers Gap, most of the unidentifiable murid skulls were of juveniles and lacked teeth, and were almost certainly M. musculus. The specimens of Notomys have been classified as N. alexis, but it is very difficult to distinguish Table 3. Prey items in barn owl pellets from study sites west of Lake Eyre Values are numbers and percentages of individuals, with percentages of prey units (PU)

Algebuckinna

Coober Pedy

Granite Downs

Rodentia L. forresti N. alexisA P. hermannsbuvgensis R. villosissimus M. rnusculus Unidentifiable Chiroptera Nyctophilus sp. Unidentifiable Marsupialia S. crassicaudata S. macroura S. ooldea Lacertilia Anura No. of pellets

21

59

64

Kenmore Park

Warrina

William Creek

Rodentia L, forresti N . alexisA P,hermannsburgensis R. villosissimus M. musculus Unidentifiable Chiroptera Nyctophilus sp. Unidentifiable Marsupialia S. crassicaudata S,macroura S. ooldea

No. of pellets A

May include some N. juscus.

56

30

39

Probably a gekkonid.

crania of this species from those of N. fuscus and only a little less difficult to distinguish crania of these two species from those of N. cervinus (see Morton et al. 1977). The identifications of Notomys specimens, therefore, must be regarded as tentative, The rabbit skull found in the Etadunna sample was that of a juvenile,


The only bird remains that were identifiable were two from the Mount King sample (Cinclorhamphus mathewsi and Glossopsitta sp., probably porphyrocephala). The frog in the Granite Downs sample was Neobatrachus centralis, and those from Mount King comprised two Limnodynastes tasmaniensis and two unidentifiable. Orthopterans or beetles were found in pellets from five of the sites. Table 4. Numbers of vertebrate prey items in barn owl pellets Site

Algebuckinna Coober Pedy Esudina Etadunna Fowlers Gap Granite Downs Kenmore Park Lake Harry Mount King Warrina William Creek

No. of pellets

Mean wt of pellets (g)

Mean No. of prey individuals per pellet

Mean No. of prey units per pellet

21 59 31 129 303 64 56 48 246 30 39

4.0 3.7 3.7 5.6 4.1 4.2 3.9 3.3 3.6 3.9 4.6

0.8 1.7 4.5 1.9 4.7 3.7 3.6 2.1 3.8 1.7 4.3

3.4 3.2 4.5 5.4 4.7 4.5 4.1 3.9 4.2 4.2 5.0

Molar wear class

Fig. 4. Percentage of individuals of Rattus villosissirnus, from barn owl pellets, in eight classes of molar wear. Solid blocks, Etadunna (n = 89); open blocks, the combined sites Algebuckinna, Coober Pedy, Granite Downs, Lake Harry, Warrina and William Creek (n = 62).

The numbers of vertebrate prey items and of prey units per pellet at each site are shown in Table 4; there is considerable variation ( > 2 prey units) in the number of prey units per pellet. At the site with the greatest number of prey units per pellet (Etadunna), barn owl nests were found; the frequent screeching of the owls was also indicative of breeding activity. It was suggested above that barn owls bred at Fowlers Gap at a time when the number of grey units per pellet was > 5.


201

The ages (in terms of extent of molar wear) of R. villosissimus specimens from the owl pellets are shown in Fig. 4. The results indicate that barn owls capture many adult R. villosissimus (molar wear classes 5-S), but because the age structures of the prey populations are unknown it is impossible to determine whether any age group was selectively preyed upon. A check was made on the validity of using cranial remains to determine the number of prey items eaten by the barn owls; Medway and Chong (1970) reported that large prey items may be decapitated by barn owls, and that the heads may not be eaten. Hence, the number of crania of R. villosissimus was compared with the number of individuals as calculated from hip-bone remains. The ratios of crania to pairs of hip bones, for five sites where the number of R. villosissimus crania exceeded 10, were: Algebuckinna, 1.2; Etadunna, 1.4; Granite Downs, 0.7; Lake Harry, 1.3; Warrina, 1.1. At four sites the number of R. villosissimus individuals represented by crania was greater than that represented by hip bones; thus cranial remains provide the more accurate measure of the number of individual rats represented. Discussion If it is assumed that, of most of the pellets collected at roosts of Australian barn owls, each represents one night's hunting (Morton 1975; Morton et al. 1977), then considerable variation in the amount of food eaten per night is evident. Morton (1975) reported that in southern Victoria 3-4 prey units were eaten per night, but in arid south-western Queensland the mean number of prey units per pellet was 4.8 (Morton et al. 1977). In the present study, the mean number of prey units per pellet varied from 3.2 to 5.4 (Table 4); this variation was also reflected in the numbers of small mammals taken. However, the results obtained in these three studies show that the amount of food and the number of prey eaten by Australian barn owls are within the ranges reported for other races of T. alba (Wallace 1948; Glue 1967, 1974; Vernon 1972); thus, Morton's (1975) suggestion that the Australian race takes a larger number of comparatively smaller prey than do other races appears to be incorrect. In fact, the present study has shown that Australian barn owls are capable of capturing adult plague rats, which may weigh up to 280 g (Taylor and Horner 1973); this also refutes an earlier statement (Morton et al. 1977). The barn owls studied in this work were almost completely dependent upon two prey species, M. musculus and R. villosissimus, and at several of the study sites M. musculus was virtually the only prey item. Presumably the density of barn owls now reaches higher levels in these areas than it did before the advent of M. musculus, but the spread of house mice in Australia is so inadequately documented (Birch 1965) that no conclusions can be drawn. Both M. musculus and R. villosissimus undergo enormous fluctuations in abundance and dispersion (Finlayson 1939, 1961; Carstairs 1974; Watts and Aslin 1974; Newsome and Corbett 1975; this study); further, it is probable that other small mammals forming important prey of barn owls, such as Leggadina forresti at Coober Pedy and Notomys alexis at Sandringham Station in Queensland (Morton et al. 1977), also form plagues (Newsome and Corbett 1975). It is apparent, therefore, that barn owls in arid Australia are dependent upon a highly variable source of food. They breed prolifically when prey are abundant (Fleay 1968; see also Otteni et al. 1972), but their response to a rapid decrease in the availability of food is unknown. Owls became uncommon at Fowlers Gap after the


drop in abundance of M. musculus in 1975, but only one was found dead. Many probably disperse; their capacity to do so has been shown by Purchase (1972). However, it seems unlikely that the owls can predict where prey will be abundant, and many presumably perish through starvation. This study, in addition to those of Morton (1975) and Morton et al. (1977), has shown that very few dasyurid marsupials are eaten by barn owls; the ratio of dasyurids to rodents in the pellets was always about 1 : 20. It is difficult to compare the densities of rodents and dasyurids because many of the latter seem to be shy of traps, but this ratio probably reflects the relative abundance of the two groups. Nevertheless, selective predation by barn owls on certain species of small mammal has been demonstrated (e.g. Pearson and Pearson 1947; Fast and Ambrose 1976; Fulk 1976), and at first sight such selective predation seems to have occurred at Fowlers Gap. The dasyurid Sminthopsis crassicaudata was never a major prey item (Table I), even though it was more common than M. musculus in the plains habitat, except during the brief irruption of the latter species in late 1974 and early 1975 (Fig. 2). However, this may be due to the owls' hunting pattern rather than to selection of M. musculus over S. crassicaudata. Owls hunting primarily over creek beds and drainage channels rather than plains habitat would be unlikely to capture many S. crassicaudata because this species rarely occurs in such habitats (Denny 1975; Morton 1978). This suggestion is supported by the fact that the creek-dwelling dasyurid Planigale gilesi became relatively common in the diet of barn owls at Fowlers Gap in late 1975 (Table 1). The Mount King data provide further evidence of selection of habitat for hunting by barn owls. Here, barn owls captured nearly twice as many individuals of the primarily creek-dwelling dasyurids (P. gilesi and S. macroura) as of the plainsdwelling dasyurids (P. tenuirostris and S. crassicaudata). The area of habitat suitable for the first two species at this site is comparatively small because of their confinement to thicker vegetation along the creeks, but S. crassicaudata and P. tenuirostris appear to be widespread over the surrounding alluvial plain (Denny 1975; Morton, unpublished observations). It seems very likely that the barn owls concentrated their hunting along the creeks. In contrast, at Coober Pedy the two most important prey species (L. forresti and N. alexis) probably occur primarily in open plains habitat; hence, the barn owls must have hunted away from the creeks. At Sandringham Station, the array of creekand plain-inhabiting small mammals captured suggested that the owls did not confine their hunting to any specific habitat (Morton et al. 1977). This investigation has shown that barn owls in the Australian arid zone are opportunistic predators upon a variety of small mammals in a range of different habitats. The variable rainfall in this region results in wide fluctuations in the abundance of small mammals, and barn owls are dependent upon rodents that are usually rare and undergo infrequent irruptions. The owls seem to capitalize on such irruptions by breeding, but they do not switch to other prey when the rodents again become rare, and in consequence they probably suffer severe mortality. Australian barn owls do not appear to have diverged from the specialization on small mammal prey that characterizes other races of this species (Honer 1963; Earhart and Johnson 1970; Vernon 1972; Glue 1974); hence, adaptation of the barn owl to arid Australia has not involved a broadening of the prey spectrum.


Acknowledgments We are grateful to Mr J. A. Mahoney, Dr M. Archer and Dr P. Crabb for examining small mammal remains and generously allowing us to use their unpublished criteria for identification. We are indebted to the following people for their help with other taxonomic groups: Mr M. J. Tyler (frogs); Mr A. R. McEvey (birds); Mr A. J. Coventry (lizards); and Mr E. Hamilton-Smith (bats). S.R.M. is grateful to the Fowlers Gap Management Committee for permission to work on the Station; to the many people, especially Mr D. Read, who assisted with trapping and the location of roosts; to Dr and Mrs M. J. S. Denny for their help and hospitality at Mount King; and to Faye Alexander for her help and companionship in the field. Work at Fowlers Gap Station was made possible by grants from the M. A. Ingram Trust. References Birch, L. C. (1965). Evolutionary opportunity for insects and mammals in Australia. In 'The Genetics of Colonizing Species'. (Eds H. G. Baker and G. L. Stebbins.) pp. 197-211. (Academic Press: New York.) Bunn, D. S., and Warburton, A. B. (1977). Observations on breeding barn owls. Br. Birds 70, 246-56. Carstairs, J. L. (1974). The distribution of Rattus villosissimus (Waite) during plague and nonplague years. Aust. Wildl. Res. 1, 95-106. Cox, J. B. (1976). Grey grasswrens and grass owls at Goyder's Lagoon, South Australia. S. Aust. Ovnithol. 27, 96-100. Denny, M. J. S. (1975). Mammals of Sturt National Park, Tibooburra, New South Wales. Aust. 2001.18, 179-95. Dwyer, P. D., and Willmer, J. B. (1975). Eastern grass owl at Cooloola. Sunbird 6, 99-100. Earhart, C. M., and Johnson, N. K. (1970). Size dimorphism and food habits of North American owls. Condor 72, 251-64. Fast, S. J., and Ambrose, H. W. (1976). Prey preference and hunting habitat selection in the barn owl. Am. Midl. Nut. 96, 503-7. Finlayson, H. H. (1939). On mammals from the Lake Eyre basin. Part 5. General remarks on the increase of murids and their population movements in the Lake Eyre basin during the years 19301936. Trans. R. Soc. S. Aust. 63, 348-53. Finlayson, H. H. (1961). On central Australian mammals. Part IV. The distribution and status of central Australian species. Rec. S. Aust. Mus. 14, 141-91. Fleay, D. (1968). 'Nightwatchmen of Bush and Plain. Australian Owls and Owl-like Birds.' (Jacaranda Press: Brisbane.) Fulk, G. W. (1976). Owl predation and rodent mortality: a case study. Mammalia 40, 423-7. Glue, D. E. (1967). Prey taken by the barn owl in England and Wales. Bird Study 14, 169-83. Glue, D. E. (1974). Food of the barn owl in Britain and Ireland. Bird Study 21, 200-10. Honer, M. R. (1963). Observations on the barn owl (Tyto alba guttata) in the Netherlands in relation to its ecology and population fluctuations. Ardea 51, 158-95. Mayr, E. (1944). Timor and the colonization of Australia by birds. Emu 44, 113-30. Medway, Lord, and Chong, Y. G. (1970). Barn owl pellets from Kulai, Johore. Malay. Nut. J. 23, 171-2. Mees, G. F. (1964). A revision of the Australian owls (Strigidae and Tytonidae). 2001.Verh.Rijksmus. Nat. Hist. Leiden 65, 1-62. Morton, S. R. (1975). The diet of the barn owl Tyto alba in southern Victoria. Emu 75, 31-4. Morton, S . R. (1978). An ecological study of Sminthopsis crassicaudata (Marsupialia : Dasyuridae) I. Distribution, study areas and methods. Aust. Wildl. Res. 5, 151-62. Morton, S. R., Happold, M., Lee, A. K., and MacMillen, R. E. (1977). The diet of the barn owl, Tyto alba, in south-western Queensland. Aust. Wildl. Res. 4, 91-7.


Newsome, A. E., and Corbett, L. K. (1975). Outbreaks of rodents in semi-arid and arid Australia: causes, preventions and evolutionary considerations. In 'Rodents in Desert Environments'. (Eds I. Prakash and P. K. Ghosh.) pp. 117-53. (W. Junk: The Hague.) Otteni, L. C., Bolen E. G., and Cottam, C. (1972). Predator-prey relationships and reproduction of the barn owl in southern Texas. Wlson Bull. 84, 434-48. Parker, S. A. (1977). The distribution and occurrence in South Australia of owls of the genus Tyto. S. Aust. Ornithol. 27, 207-15. Pearson, 0. P., and Pearson, A. K. (1947). Owl predation in Pennsylvania, with notes on the small mammals of Delaware County. J . Mammal. 28, 137-47. Purchase, D. (1972). Report on the banding of barn owls. Aust. Bird Bander 10, 74-5. Raczynski, J., and Ruprecht, A. L. (1974). The effect of digestion on the osteological composition of owl pellets. Acta Ornithol. (Warsaw) 14, 25-38. Southern, H. N. (1954). Tawny owls and their prey. Ibis 96, 384-410. Taylor, J. M., and Horner, B. E. (1973). Results of the Archbold Expeditions. No. 98. Systematics of native Australian Rattus (Rodentia, Muridae). Bull. Am. Mus. Nat. Hist. 150, 1-130. Vernon, C. J. (1972). An analysis of owl pellets collected in southern Africa. Ostrich 43, 109-24. Wallace, G. J. (1948). The barn owl in Michigan. Mich. Agric. Exp. Stn Tech. Bull. No. 208. Warneke, R. M. (1971). A field study of the Australian bush rat, Rattus fuscipes Waterhouse (Rodentia : Muridae). Victoria Fish. Wildl. Dep. Wildl. Contrib. No. 14. Watts, C. H. S., and Aslin, H. J. (1974). Notes on the small mammals of north-eastern South Australia and south-western Queensland. Trans. R. Soc. S. Aust. 98, 61-9.

Manuscript received 30 October 1978