density and distribution of the australian bustard ardeotis australis

Biological Conservation 35 (1986) 259-267

Density and Distribution of the Australian Bustard Ardeotis australis

D a v i d Grice, Graeme Caughley & Jeff Short

Division of Wildlife and Rangelands Research, CSIRO, PO Box 84, Lyneham, ACT 2602, Australia

A B S T R A C T

The density and distribution o f the Australian bustard Ardeotis australis was determined f rom observations made during aerial surveys o f kangaroos. Bustards were most abundant in the northern half o f Australia, particularly on mitchell grass plains. They were rare or absent in much of southern Australia, on parts o f which they were reported as common 100 years ago. Bustard density and distribution seems to be influenced by vegetation structure, land use and predation.

INTRODUCTION

The majority of the 22 species of bustards in the world have been described either as threatened or as suffering from large-scale contrac- tions in distribution (Isakov, 1974; Collar, 1979; Cramp & Simmons, 1980; Farrell, 1984). Much of this decline occurred this century and has been attributed largely to hunting and to habitat change brought about by overgrazing and intensive agricultural practices (Isakov, 1974; Cramp & Simmons, 1980).

We examine the distribution and density of the Australian bustard Ardeotis austral& (Gray). Like all bustard species, the Australian bustard is a ground-nesting bird that eats plant material (green herbage, seeds and fruits), invertebrates and some small vertebrates (Boehm, 1947;

259

Biol. Conserv. 0006-3207/86/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

260 David Grice, Graeme Caughley, Jeff Short

Cramp & Simmons, 1980). It is similar in habit and form to the great Indian bustard A. nigriceps (Vigors) and both species are probably subspecies of the kori bustard A. kori (Burchell) (R. Schodde, pers. comm.). A. australis occurs only in southern New Guinea and the Australian mainland, where it is a protected species throughout the year. Before the early 1900s flocks of up to 1000 were reported in grassland and low shrubland areas throughout Australia (Boehm, 1947; Frith, 1973; Downes, 1982). Today, A. australis is rarely seen in the southern parts of its range (Downes, 1982; Blakers et al., 1984).

METHODS

Data on the relative abundance of A. australis on the Australian mainland were collected in winter during aerial surveys of kangaroos between 1976 and 1984. Over 90% of these data were collected between 1980 and 1984 (Fig. 1). The 1980 data for New South Wales were provided by J. Caughley from surveys by the New South Wales National Parks and Wildlife Service.

Bustards are easily identified from the air. The noise of the aircraft usually flushes the birds, which then fly low over the ground with a distinctive, slow, measured wing beat. The observer's eye is immediately attracted by the broad brown and buff wings, each with a prominent powder-blue ellipse on the upper surface. The search image is conspicu- ous and unmistakable. In contrast, that of a standing bird is less distinct. The observer's eye is drawn to the vertical white neck, contrasting with the black-capped head and brown body. The image lacks the conspicuousness of that of the bird in flight and we suspect many of the birds not flushed are not seen.

Sampling intensity per degree block in the sheep pastoral and agricultural land use zones was 0.7%, except in New South Wales and southern South Australia where it was approximately 1-3%. Outside these areas the sampling intensity was 0.35%.

All transects were flown at a height of 76m and at a speed of 185kmh -~. Two observers, one each side of a Cessna 182, 206 or 210, each scanned a 200 m strip on their side of the aircraft. Width of transect was demarcated for each observer by a streamer attached to the wing strut. After each 5 km of track (a distance covered in 97 s) an electronic timer triggered a whistle that lasted 7 s. In those 7s each observer

The Australian bustard 261

az7

NS

Hlamersley Range 1982

1981

Taeami Desert

1981 INS i

r Bedour lee

I

1980

l a cka l l

1982

Fig. 1.

Q t

Survey areas and their time of completion. NS, not surveyed.

recorded the number of kangaroos and the presence or absence of bustards in the 1 km 2 sampling unit (5 km long by 0.2 km wide).

Data for a series of sampling units were expressed as percent frequency per km 2 for each degree block (1 ° latitude by I ° longitude). This provided an index of relative abundance.

RESULTS

An index of density and the distribution of bustards in the surveyed areas of Australia is shown in Fig. 2.

Most birds were seen in the northern half of Australia with the highest concentrations on the mitchell grass Astrebela spp. plains near Julia


Creek in Queensland. Other high concentrations in Queensland occurred in two broad bands from near Julia Creek in the north for 500 km south-west to Bedourie and 550 km south-east to Blackall.

In Western Australia high concentrations occurred in an area 100- 200 km south-east of Broome and east of the Hamersley Range. In the

a:7

NS

N.T.

W.A.

Fig. 2.

S,A.

i ~ N.S.W.

S % frequency / km 2 ,~o

l I None seen ¢c,' I-4 4-10

> 1 6 0

NS Not surveyed ~ *

The d is t r ibut ion o f bustard density in the survey area.

Northern Territory high concentrations occurred in a broad band 300 km long between Lake Mackay and the Tanami Desert.

Bustards were generally confined to areas where the upper canopy cover was less than 10% or where the upper canopy was less than 2 m in height (Figs 2 and 3). The highest concentrations were in or adjacent to areas where grasses dominated.


Fig. 3.

m Tallest stratum > 10% canopy cover o r • 2m high

' ~ leared for intensive landuse practices

b

V Broad categories of vegetation structure adapted from Division of National

Mapping 0976, 1979).

DISCUSSION

A five-year survey of birds in Australia completed in 1981 by the Royal Australasian Ornithologists' Union (RAOU) found that bustards are now absent in the south-east regions of Australia (Blakers et al., 1984). Bustards are rarely seen now in New South Wales, Victoria, and the southern parts of Queensland and South Australia, whereas in the late 1800s flocks of 30 to 1000 could be seen in parts of these states (Berney, 1936; Boehm, 1947).

The majority of the RAOU ground survey records of bustards were in the northern half of Australia (Fig. 4), a pattern reflected by the density indices we obtained from aerial surveys (Fig. 2). However, the


Fig. 4.

v V

11-40 0

The distribution of bustard reporting rates adapted from Blakers et al. (1984).

results of the two surveys differ in detail, probably reflecting the RAOU data being accumulated in each degree block over a five-year period with degree blocks revisited a varying number of times and in varying seasons and years. The RAOU data are smoothed by repeated surveying and are influenced to some extent by variability in method and efficiency among the many observers. Their data provide a better indication of the total range of the species. By comparison, our data provide a snapshot for each degree block. Each degree block is surveyed on a single winter day, although the day and the year may differ between blocks. Our data may be viewed more as an instantaneous pattern of density derived from tightly standardised observations by a few observers.

The aerial survey did not cover the eastern tablelands of Queensland


where RAOU records indicate high reporting rates (Fig. 4). The perma- nency of this area as suitable bustard habitat is questionable. Blakers et al. (1984) suggested that the range of bustards may have expanded into the area as they had previously been absent there for many years or known only as a rarity. Bustards may have irrupted recently into parts of this area following clearing of woodland and the planting of tussock grass pastures (B. Brown, pers. comm.). Temporary increases in areas of previously unsuitable habitat have occurred before in Australia (Carter, 1923; Downes, 1982) and with other bustard species elsewhere in the world. In Europe during the 18th century there were temporary increases in numbers and distribution of the great bustard Otis tarda (L.) following the clearing of forest (Isakov, 1974). The increase in numbers was followed by a marked decline as the intensity of land use increased and more of the cleared areas were continuously cultivated (Isakov, 1974).

Vegetation structure seems to have an overriding influence on bustard distribution, with higher densities being confined to areas in or adjacent to open country (Figs 2 and 3). Downes (1982) suggests that the daily and seasonal movement of A. australis between open plains and adjacent areas of cover is a key feature of the habitat requirements of the species.

Various authors suggest that bustard numbers are limited by: alter- ation of habitat by rabbits, sheep grazing and intensive land use practices; physical disturbance of breeding adults; fox predation; hunting; and poisoning by rabbit baits (Boehm, 1947; Frith, 1973; Dowries, 1982). The bustard's low breeding rate might also be implicated by limiting rate of increase after a reduction in numbers. Females do not mature until approximately four years of age and then only one or two eggs are laid each breeding season (Anon., 1971). Males are not sexually mature until five to seven years of age (Anon., 1971; Appayya, 1982).

The limiting effects of foxes and other predators have been considered of secondary importance, their impact probably increasing when breeding success is already depressed (Berney, 1936; Boehm, 1947; Downes, 1982). We found few bustards where foxes are common. In Australia the main concentrations of foxes are in the south, particularly the south- east, the reverse of the bustard distribution.

Contrary to what was suggested by Frith (1973), high densities of bustards occur in parts of the sheep pastoral zone. If extensive sheep grazing does limit bustard numbers, then its effect is not yet evident in the northern half of the sheep pastoral zone of Queensland. These


northern areas of the sheep pastoral zone were not affected by the rabbit plagues of the late 1800s to mid-1900s which, in combination with sheep, severely overgrazed extensive areas in the south (Rolls, 1977). Many of the tussock-forming perennial grasses, with their suite of herbs in between, disappeared or were much reduced in density (Downes, 1982). This may have reduced the amount of cover and feed available to the bustard and permanently reduced bustard densities in the southern sheep pastoral areas. Today, the highest densities of rabbits are still south of the Tropic of Capricorn, the reverse o f bustard distribution. A more direct effect of the earlier rabbit plagues was the widespread use of poisons to control rabbits (Rolls, 1977). Bustards were poisoned in their hundreds in New South Wales (Rolls, 1977; Austin, 1907) and in South Australia (Boehm, 1947). The influence this has had on present distribution is not known.

Intensive land-use practices have been implicated in the decline of A. australis in parts of Australia (Frith, 1973; Downes, 1982), the marked decline of O. tarda in Europe (Isakov, 1974) and the near extinction of A. nigriceps in India (King, 1981). Both our survey results and those of the RAOU corroborate this for A. australis in Australia. A. australis is absent in the majority of areas of intensive land-use practices (Figs 2, 3 and 4). The influence of intensive land use on bustards depends on the time of cultivation relative to the breeding season, the extent and duration of the fallow period in crop rotation, the extent pesticides are used and the amount of disturbance caused by increased human activity (Isakov, 1974).

A major problem in interpreting the current geographic pattern of bustard density is that the 19th-century pattern, with which it might usefully be compared, is known only from isolated records. We cannot determine whether there has been a general lowering of the 19th-century densities or whether densities in the north remain much the same. Subsequent aerial surveys will enable monitoring of future trends in density and changes in distribution.

AC KNOW L E DGEM ENTS

We thank B. K. Brown for assistance in the field, and P. Fullagar and R. Schodde for criticising a previous draft of this paper.


R E F E R E N C E S

Anon. (1971). Our diminishing heritage. S.W.A.N.S., 2, 40-1. Appayya, M. K. (1982). Breeding of bustards: an observation in Australia. J.

Bomb. nat. hist. Soc., 79, 195-7. Austin, T. B. (1907). Field notes on birds from Talbragar River, New South

Wales. Emu, 7, 74-9. Berney, F. L. (1936). The bustard in Queensland. Emu, 36, 4-9. Blakers, M., Davies, S. J. J. F. & Reilly, P. N. (eds) (1984). The atlas of

Australian birds. Melbourne, Melbourne University Press. Boehm, E. F. (1947). The Australian bustard: with special reference to its past

and present status in South Australia. S. Aust. Ornithol., 18, 37-40. Carter, T. (1923). Birds of the Broome Hill district. Emu, 23, 125-42. Collar, N. J. (1979). Bustard group general report. In Bulletin of the International

Council for Bird Preservation, No. 13, ed. by P. Barclay-Smith and R. D. Chancellor, 129-34. London, International Council for Bird Preservation.

Cramp, S. & Simmons, K. E. L. (eds) (1980). Handbook of the birds of the Middle East and North Africa. The birds of the Western Palearctic, 2, Hawks to bustards. London, Oxford University Press.

Division of National Mapping (1976). Atlas of Australian resources, second series, Natural vegetation.

Division of National Mapping (1979). Atlas of Australian resources, third series, Land use.

Downes, M. (undated [1982]). Re-establishment of the bustard in Victoria. In Wildlife management in the '80s, ed. by T. Riney, 227-39. Proc. Conf. Field and Game Federation of Australia and Graduate School of Environmental Science, Monash University, 27-29 November 1981.

Farrell, A. (ed.) (1984). WWF Yearbook 1983/84. Gland, WWF-International. Frith, H. J. (1973). Wildlife conservation. Sydney, Angus and Robertson. Isakov, Yu. A. (1974). Present distribution and population status of the great

bustard, Otis tarda Linnaeus. J. Bomb. nat. hist. Soc., 71,433-44. King, W. B. (ed.) (1981). Endangered birds of the world." the ICBt a bird red data

book. Washington, Smithsonian Institution Press. Rolls, E. (1977). They all ran wild." the story of pests on the land in Australia.

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density and distribution of the australian bustard ardeotis australis

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