Koobabbie is located in the centre of the Northern Agricultural Catchment Council (NACC) area. It is an important area for the conservation of the avifauna of the NACC area (Saunders and Doley 2013). Two hundred and fifty-four hectares of the property are mapped and listed as an important bird area by BirdLife Australia (http://birdlife.org.au/projects/important-bird-areas/iba-maps accessed 9 January 2014) for the support of “up to 32 breeding pairs of the endangered Carnaby’s Black-Cockatoo which nest in Salmon Gum on the property”, and three other biome-restricted species [Western Corella, Regent Parrot (Polytelis anthopeplus) and Blue-breasted Fairy-wren (Malurus pulcherrimus)].

Koobabbie has a Mediterranean climate of hot, dry summers and cool, wet winters. The property has a mean annual rainfall of 337 mm (1911–2011 rainfall records from Koobabbie) with 76% of the rainfall occurring between April and September. From 1987 to 2011, total annual rainfall varied from 198.6–560.4 mm. January is the hottest month with a mean daily maximum temperature 36.0 °C and minimum 18.5 °C (data from nearest temperature recording station at Carnamah, 39 km north-west of Koobabbie, Australian Bureau of Meteorology website http://www.bom.gov.au accessed 9^th August 2012) and July is the coolest (15.6 °C and 4.6 °C).

The property is 7,173 ha of which 41.5% still retains native vegetation, although 58% of this uncleared land is a major salt lake complex in the property’s northeast (Figure 1). Of particular relevance to Carnaby’s Cockatoo are the extensive woodland strips that occur throughout the property. These remnants of the original woodlands found in the region are dominated by Salmon Gum (Eucalyptus salmonophloia), Gimlet (E. salubris) and York Gum (E. loxophleba). They provide hollows used for nest sites by the cockatoos. The remnant vegetation on the deep, yellow, sandy soils mainly found west of Koobabbie includes species of Banksia, Grevillea and Hakea that provide food for the birds. Only 15% of the area within 12 km radius of Koobabbie remains covered with native vegetation (Saunders et al. 2014b).

Figure 1.

Koobabbie showing property boundary, remnant vegetation, and revegetation 2007–2011).

Between 2007 and 2011, 164.6 ha of revegetation was carried out on Koobabbie (Figure 1). The revegetation is of native species of local provenance. However, it has little value for Carnaby’s Cockatoo. The birds nest in the woodland on Koobabbie and forage to the west of the property. Little of the revegetation has been placed where the birds forage and the red clay loam is not suitable for growing species suitable as sources of food. York Gum is the principal species planted in many areas and understorey plants are Melaleuca and Acacia with no Proteaceae. In slightly saline areas Atriplex amnicola has been planted to provide grazing for sheep.

Detailed descriptions of Koobabbie and its management are provided in Doley (1995, 2003) and Saunders and Doley (2013).

Carnaby’s Cockatoo, a large black cockatoo with a distinctive white tail band, is endemic to southwestern Australia. It has been extensively studied (Saunders and Ingram 1998, Saunders et al. 2014b), with one population at Coomallo Creek in the northern wheatbelt of WA being studied in detail from 1969 to the present. In the late 1960s the species was classified as vermin due to its impact on primary production with a bounty on its bill. By the mid-1980s, as a result of destruction and fragmentation of its habitat, it had declined in range and abundance, and became the subject of conservation concern (Saunders 1982, 1990). Currently it is listed as endangered under the Australian Environment Protection and Biodiversity Conservation Act 1999 and under IUCN Red List category and criteria (IUCN 2014). It is specially protected as “Fauna that is rare or likely to become extinct” in Schedule 1 of the Western Australian Wildlife Conservation Specially Protected Fauna Notice 2013 under the Wildlife Conservation Act 1950. It is the subject of a recovery plan which has recently been revised (Department of Environment and Conservation 2012).

From May 1987 to the present, the Doleys recorded every species of bird seen on Koobabbie each week. These data were presence only; for example, one Wedge-tailed Eagle (Aquila audax) seen on one occasion during one week was recorded as a tick in the data sheet for that week, as were 100 Western Corella seen every day of the week. In addition, they made notes relating to birds of particular interest. These data and the accompanying notes may be seen on a Supplementary Table to Saunders and Doley (2013) at the journal website http://pcb.murdoch.edu.au. Records of the occurrence of Carnaby’s Cockatoo on the property, together with notes relating to their nesting, behaviour and flock sizes are available from the second half of 1987 to the end of 2013; a period of 28 years.

From the breeding season of 2003, staff or volunteers from Birds Australia visited Koobabbie once each year, sometime from late September to early November, and searched for active Carnaby’s Cockatoo nests. This was done by looking in each hollow known by the Doleys to be used by Carnaby’s Cockatoo, or trees thought to contain a hollow of sufficient size to be suitable for the birds. Any trees with female Carnaby’s Cockatoo leaving the hollows or with nestlings or eggs were recorded for subsequent examination. This was carried out by staff from the Department of Conservation and Land Management who recorded the contents of the hollow. If nestlings were large enough to be handled (>3 weeks old), they were measured (length of folded left wing [mm] and body mass [g]), banded (Australian Bird and Bat Banding Scheme bands), and several breast feathers removed for genetic analyses.

The following dimensions were recorded from each hollow used by Carnaby’s Cockatoo: height of entrance above ground level (m); diameter of the entrance to the hollow (if circular or the width of the narrowest side, if not circular) (m); depth of the hollow (m); and diameter of the floor of the nest chamber (m). The species of tree providing the hollow was also noted.

Nestlings were aged by comparing the length of their folded left wing (mm) against a reference curve of the length of the folded left wing of known age nestlings from Coomallo Creek, using the method described by Saunders (1986). Saunders (1986) regarded the population at Coomallo Creek as the one in which the nestlings were in the healthiest condition. The accuracy of aging nestlings using this method was ± 4 days applied to nestlings aged around 31 days and ± 6 days applied to nestlings around 64 days. This is a 13% variation at 31 days and 9% at 64 days. This is a reasonable error over a nestling period of more than 70 days. From the nestlings’ estimated ages, egg laying dates were extrapolated.

Nestlings were sexed based on DNA analysis. DNA-based sex identification targeted the CHD-W and CHD-Z genes located on the sex chromosomes (female, ZW; male, ZZ) using modified primers of Griffiths et al. (1998). Kinship (relatedness) analyses followed White et al. (2009, 2012) to determine how many and how often individual breeding females nested in hollows on Koobabbie.

Saunders (1982) developed a growth curve for Carnaby’s Cockatoo based on the relationship between estimated age and expected body mass (g), with standard deviations that ranged from ±20.7% of body mass for nestlings 18 days old, to ±7.4% of body mass for nestlings 68 days old. This reference curve was constructed from measurements of nestlings at Coomallo Creek (1970–1976), 71 km west of Koobabbie. By comparing the observed body mass of nestlings at Koobabbie with their expected body mass relative to their estimated ages based on the measurement of the folded left wing, it was possible to determine which nestlings were in poor condition. Poor condition is defined here as being those nestlings whose body mass was more than one standard deviation below the expected mean body mass for their estimated age (Saunders 1986, Saunders et al. 2014b).

Nearest neighbour analyses, using the distance between the waypoints function on OziExplorer GPS Mapping Software, were carried out on all hollow trees used by Carnaby’s Cockatoo in the breeding seasons of 2006, 2007 and 2008. These years were chosen because they had the largest number of breeding attempts. In addition, nearest neighbour analyses were carried out on nesting attempts commenced in a series of two-week periods in each of these years. That is, distances (m) to the nearest hollows occupied during weeks 1 and 2 of the breeding season, then weeks 2 and 3, 3 and 4, etc. were calculated. The two-week period was chosen as Saunders (1982) pointed out that females selecting and preparing nest hollows will not tolerate another female prospecting for a nest hollow nearby, but once the female has laid and is incubating, she will tolerate other females nesting as close as hollows in the same tree.

On 1 April 2004, seven artificial hollows were erected on Koobabbie. These hollows were constructed of sections of fallen hollow Salmon Gum, with a floor of metal covered with decayed heartwood material from fallen trees, a wooden roof, and an entrance opening to the top and side of the hollow. No measurements were taken of the internal dimensions of artificial hollows, but they were approximately 600–700 mm deep, with an internal diameter of at least 250 mm. They were supported on a 5.8 m length of 100 mm diameter galvanised water pipe. The pipe was supported on a galvanised iron bracket set in concrete with two bolts through the bracket and the pipe, so that by removing one bolt the pipe may be lowered using a front end loader or block and tackle should the hollow need repair. There were several constraints on placement of the artificial hollows; chance of use by Carnabys Cockatoo, vehicle access for erection, and clearance from neighbouring trees. All artificial hollows were erected in woodland known to be frequented by the cockatoos for nesting. Each hollow required access for a tractor with post-hole auger, an eight-tonne truck with 5.8 m of pole and hollow, and a front end loader with a loader bucket. The hollows were placed sufficiently far from neighbouring trees to ensure they would not be damaged should any of those trees fall. The artificial hollows were monitored during the same period natural hollows were monitored.

Galah and Western Corella compete for hollows with Carnaby’s Cockatoo, and in some cases destroy Carnaby’s Cockatoo eggs and take over hollows (Saunders 1979, 1982, Alison Doley and Rick Dawson pers.obs.). Since November 1989, with appropriate licences, the Doleys have arranged for the destruction of any Galah or Western Corella investigating hollows used by Carnaby’s Cockatoo, or in the vicinity of such hollows.

Feral Cats (Felis catus) are known to climb trees and prey on cockatoos breeding in hollows, killing nestlings and adults (Saunders 2006). In view of the threats Cats posed to breeding cockatoos, they are controlled on the property opportunistically throughout the year.

Carnaby’s Cockatoo is a regular breeding visitor to Koobabbie, arriving sometime between the first week in July and the third week in August (median arrival week the last week in July) (Table 1). The birds remain around the property for a period of 22 to 34 weeks (median length of stay 27 weeks) (Table 1), departing sometime between the second week in January and the third week in March (median departure week first week in February) (Table 1). The arrival week of the birds at Koobabbie was significantly correlated (R² = 0.166; p = 0.035) with that year’s total Austral autumn rainfall (March to May inclusive) on the coastal plain where they spend the non-breeding season (Figure 2). The wetter the autumn, the earlier the birds arrive at Koobabbie. For example, an increase in annual autumn rainfall of 75 mm advances arrival time by about one week.

Figure 2.

Correlation between week of arrival of Carnaby’s Cockatoo at Koobabbie each year (1987–2013), and total autumn rainfall (March-May) each year at Badgingarra Research Station. Rainfall from this station has been used as the birds spend their non-breeding season on the coastal plain from Badgingarra northwards.

The number of known breeding attempts by Carnaby’s Cockatoo at Koobabbie over the 11 years ranged from 18 when monitoring by Birds Australia commenced in 2003, to a maximum of 27 in 2008 (Figure 3). From 2003 to 2013 the number of natural and artificial hollows being monitored increased from 26 to 89. From 2005 to 2013, the monitoring effort for nesting birds was similar. As there were only two monitoring visits each year, these figures represent a minimum number of breeding attempts each year. Accordingly, hollows from which females were flushed on the first visit, that were empty on the second visit, may not have been used, may have been used unsuccessfully, and no evidence remained of the failure, or may have been successful by the time the hollow was checked, but again with no evidence of use.

Figure 3.

Number of known breeding attempts by Carnaby’s Cockatoo at Koobabbie 2003–2013 (diamonds). Known breeding attempts were established by examining the contents of the hollow. Number of female Carnaby’s Cockatoo flushed from tree hollows (squares), but the contents were not checked then, or were checked subsequently and no conclusive evidence of a nesting attempt was obtained.

The breeding population in 2009 was less than half that of 2008 due to the death of a number of females, and possibly males. On 30 September 2009, a male Carnaby’s Cockatoo was found dragging itself along the ground with its beak. It weighed 434 g, only two-thirds of the average body mass of adult males (Saunders 1974). On 23 October 2009, a mass death of breeding females was recorded with 11 found dead (carcases fly-blown and dried out) in their hollows, and a further five hollows were recorded with abandoned eggs. Four dead females were found on the ground in the nesting area. Dead females were distributed in hollows throughout the woodland areas of Koobabbie, as were the females who successfully bred that season.

There was a similar episode in mid-October 2012, with five females found dead on eggs in their nest hollows. Again, a male with hind limb weakness was found on the ground near the homestead. It was sent for treatment at the Perth Zoo Veterinary Department, but later died. A female found in a moribund state in a nest hollow, was sent to the Perth Zoo Veterinary Department, and later euthanized. Carnaby’s Cockatoo feathers on the ground in a belt of woodland indicated an eighth bird had died. These losses again halved the breeding population such that only five breeding attempts were recorded in 2013 (Figure 3); one of these was a second breeding attempt in a hollow where the first attempt failed (Rick Dawson pers. obs.).

Necropsies on the two birds found alive, but compromised in 2012, showed no signs of infectious disease or other significant pathological abnormalities, except for the female which had evidence of severe dehydration. Testing was negative for several infectious pathogens including Newcastle disease virus, avian influenza virus, beak and feather disease virus, avian polyomavirus, Chlamydia psittaci and avian adenovirus. Screening was negative for 72 toxins including pyrethrins, organophosphates and organochlorines, and testing for seven heavy metals did not reveal any significant findings. It is highly unlikely that food shortage was the cause of the deaths, as one of the males found alive was in good body condition, and some of the nestlings measured that were in unaffected hollows were within the healthy body mass range.

During the period 2003–2013, the earliest that egg-laying commenced was Week 31 (Jul 31–Aug 6) in 2008, and the latest Week 45 (Nov 6–12) in 2009 (Table 2). Over all years, 52.6% of eggs were laid during Weeks 35 to 37 (Aug 28–Sep 17). During the breeding seasons of 2006, 2007 and 2008 when most breeding attempts were recorded, egg-laying took place over a 7-9 week period. In the two years after the 2009 mass deaths, egg-laying also took place over a period of seven weeks (2010) and six weeks (2011).

Maximum flock sizes recorded on the property were: 91 (November 1994); 40 (December 1998); 40–60 (December 1999); 58 (October 2001); 98 (November 2004); 46 (December 2007); 26 (September 2009); 18 (September 2010); 23 (November 2011); 18 (August 2012); and 11 or 12 (September 2013).

Carnaby’s Cockatoo were recorded feeding on Wild Radish (Raphanus raphanistrum) (an agricultural weed), Wild Geranium or Corkscrew (Erodium moschatum) (an agricultural weed), Banksia prionotes, B. attenuata and Hakea preissii on three occasions, and in 2007 on Canola (Brassica spp.), an agricultural crop which was first grown in the area in 1998. DNA analyses were used to examine the intestinal content of one of the paralysed birds brought from the field in 2012. The intestines contained plant families Asteraceae, Myrtaceae, Lauraceae, Rutaceae and a clade within asterids known as lamiids.

DNA-based sex identification was carried out on 75 nestlings sampled from 2003–2010. Of these, 35 were identified as female and 40 as male (Table 3). There was no significant departure from a 1:1 sex ratio (analysis fitting a generalised linear model), which was also the case at Coomallo Creek (Saunders et al. 2013).

From the kinship analyses, fifteen breeding pairs were identified to have visited Koobabbie, at least twice, accounting for 48% of the offspring sample. Thirteen of these pairs were not recorded as breeding in consecutive years. Six pairs used the same tree at least twice.

Seventy-eight nestlings were measured and weighed at Koobabbie between 2003 and 2013 (Table 4). Of these, 13 (16.7%) were more than one standard deviation below the benchmark and deemed to be in poor condition. In 2009, the first year of the mass deaths, 60.0% of nestlings were more than one standard deviation below benchmark body mass.

Comparisons were made between data from Koobabbie and data from 963 nestlings at Coomallo Creek (1970–2013) and 73 from Manmanning (1969–1976). Of the Coomallo Creek nestlings, 110 (11.4%; annual range 0–28.6%) were more than one standard deviation below the benchmark body mass, and at Manmanning 46 (63.0%; 14.3–100%) of nestlings were more than one standard deviation below benchmark body mass.

There was a significant negative linear correlation (R² = 0.110; p = 0.004) between time after the commencement of egg-laying for the season and the percentage nestling body mass deviated from the benchmark (Figure 4). The later eggs are laid in the breeding season, the greater the chances the resulting nestlings were in poor condition.

Figure 4.

Significant correlation between the percentage of nestlings at Koobabbie whose body mass deviates from the benchmark body mass, and the time at which the egg from which they hatched was laid after the commencement of egg-laying for the season. Egg-laying commenced at Week 1.

Carnaby’s Cockatoo nested in hollows in 51 trees on Koobabbie, the dimensions of which are given in Table 5. Forty-nine were in Salmon Gum, and one each in Gimlet and York Gum. The mean depth was 1.32 m, which was similar to the depths of hollows in Salmon Gum used by Carnaby’s Cockatoo at Manmanning (Saunders 1979). The mean height of hollow entrances was 5.29 m, which was lower than the 7.38 m at Manmanning. However, the range of 2.2–8.7 m indicates that height of entrance is not of major importance; the size of the hollow in the tree is critical (Saunders et al. 2014a).

Of these hollows, 27 (52.9%) needed some form of repair, and three of the hollow-bearing trees (5.9%) had fallen over by the breeding season of 2013.

Between 2006 and 2008, the average distance between all hollow trees known to be used by Carnaby’s Cockatoo at Koobabbie varied from 130 to 180 m (Table 6). However, the average distance between hollow trees in which birds commenced laying at similar times of the breeding season was greater at 600–1050 m. The birds at Koobabbie behave in a similar manner to those at Coomallo Creek, where, over the period 1974–1976, the average distance between the nearest neighbours of all hollows used was 170 ± 10 m, and the average distance between neighbouring hollows laid in a particular week or the previous week was 800 ± 50 m.

In 2004, the installation costs of each of the seven artificial hollows were $AUD225 for materials and $AUD330 for labour.

Six were used by Carnaby’s Cockatoo at least once over the period 2004 to 2013 (Table 7); 45.7% of the 70 hollow-years they were available. Ten-percent were used by Galah, 2.9% by Barn Owl and 1.4% by Red-tailed Black Cockatoo during the hollow-years available. Artificial Hollow 3 was not used by Carnaby’s Cockatoo as the floor of the hollow was too small (Rick Dawson pers. obs.). As no follow up visits were made at the end of each breeding season, no data are available on breeding success of birds nesting in artificial hollows.

Eighty Carnaby’s Cockatoo nestlings were banded on Koobabbie between 2003 and 2013. It is not known how many of these fledged, or how many returned to breed. Two fledglings banded at Koobabbie were recorded outside their natal area. One young male was photographed in the Coomallo Creek breeding area on 12 November 2010 (Saunders et al. 2011a). This immature male appeared to be in the company of a female, in a flock that included adults and other immature birds, 71 km west-south-west of its natal area. The second was a female photographed on 12 September 2012, in a flock of up to 400 birds on native vegetation in Beekeepers Nature Reserve, 108 km west of its natal area.

Of the six cockatoo species occurring on Koobabbie, Galah were the most common; then in decreasing order, Western Corella, Red-tailed Black-Cockatoo, Carnaby’s Cockatoo, Major Mitchell’s Cockatoo and Little Corella (Alison Doley pers.obs.). Galah, Western Corella, Red-tailed Black-Cockatoo and Major Mitchell’s Cockatoo were resident on Koobabbie, and Little Corella a vagrant (Saunders and Doley 2013). Galah, Little Corella and Red-tailed Black-Cockatoo were not present in the district prior to clearing of native vegetation for agriculture. These are birds of the arid zone that have extended their range into southwestern Australia as a result of development of agriculture with the provision of grains, agricultural weeds and water for live-stock (Saunders et al. 1985, Saunders and Ingram 1995). Western Corella has increased in numbers on Koobabbie with the development of agriculture (Alison Doley pers. obs.), and is present in the district in the thousands.

During September 1995, the Doleys recorded incidents in which Western Corella usurped Carnaby’s Cockatoo at five nest hollows known to be used by the latter. In one hollow three Western Corella eggs were removed from the hollow and Carnaby’s Cockatoo reclaimed the hollow, and laid two eggs. Subsequently a Western Corella was seen investigating the hollow, and the Doleys found both black cockatoo eggs had holes in the side consistent with them being pecked open. Between 8 February 1997 and 22 March 2014, 11,741 Galah and 4,591 Western Corella were shot on Koobabbie in the vicinity of known black cockatoo nesting hollows.

In October 2007, a female Carnaby’s Cockatoo was sitting on two eggs in a hollow in a dead Salmon Gum; this nesting attempt failed as the female was killed by a Cat. Thirty-eight Cats were shot on the property.

During the 2008 breeding season, Alison Doley found Carnaby’s Cockatoo nestlings in three hollows were being adversely affected by small black ants (species unknown). This constituted 11% of the hollows known to be used by Carnaby’s Cockatoo that season. Previous observations suggested that ants only invaded nest hollows in which there was a dead nestling. In Hollow 24 on 4 December 2008, the nestling was covered in ants and showing signs of distress. The cloaca and surrounding skin were reddened and scabby. The base of the tree in which the hollow was located was sprayed with chlorpyrifos. The ants were controlled, and by 15 December the nestling had recovered. It fledged on 15 January 2009. One other nestling was found affected by ants. The surrounds of the nest hollow were sprayed, and the nestling fledged on 19 February 2009. No other cases of attack by ants on nestlings were recorded.

Carnaby’s Cockatoo has been a regular breeding visitor to Koobabbie for as long as Alison Doley can remember. Saunders et al. (2013) showed a significant correlation between the commencement of egg-laying and autumn rainfall for birds from Coomallo Creek and Manmanning in which the higher the autumn rainfall, the earlier egg-laying commenced. As data on dates of egg-laying at Koobabbie are limited, week of arrival has been used as an indicator of commencement of breeding. Over the period 1987 to 2012, at Koobabbie Carnaby’s Cockatoo conformed to this association with autumn rainfall.

It is difficult to establish a link between the numbers of Galah and Western Corella removed from the local populations, and the steady increase in nesting attempts by Carnaby’s Cockatoo. This is in part due to the fact that new nest hollows were still being located after 2005. Anecdotal evidence indicates that the program has been beneficial. For example, in the breeding season of 2007, a Carnaby’s Cockatoo commenced breeding in Artificial Hollow 7, but was usurped by a pair of Western Corella. The female Western Corella was shot. The next evening a female Carnaby’s Cockatoo was inspecting the hollow; subsequently the hollow was used, with a nestling later banded. Given that the numbers of Major Mitchell’s Cockatoo and Red-tailed Black-Cockatoo breeding at Koobabbie also increased during the same period (Alison Doley pers. obs.), it seems reasonable to conclude that controlling Galah and Western Corella improved the value of the site for the other three cockatoo species by limiting competition for hollows.

One salient point when considering competition for hollows is that unlike the other cockatoos, which only frequent the nest hollow during the breeding season, Galah guard their breeding hollow throughout the year (Rowley 1990). Galah also have an impact on availability of nest hollows through their destructive habit of “stropping” or chewing the bark away from the trunk of the tree below the nest hollow, in some cases “ring-barking” the tree. This can result in the premature death of the tree (Rowley 1990, Saunders and Ingram 1995: Plate 2).

The six largest artificial hollows were used by Carnaby’s Cockatoo during more than half of the hollow-years they were available. Natural hollows formerly used by the cockatoos that were repaired were used soon after being repaired. The rapidity with which the artificial and repaired hollows were taken up and used indicates that the provision of artificial hollows and repairs of natural hollows are useful strategies for bolstering the stock of nesting hollows.