NSW Scientific Committee - final determination
The Scientific Committee has found that:
1. The Australasian Bittern Botaurus poiciloptilus (Wagler, 1827) (family Ardeidae) is a moderately large (70 cm in length) heron-like wetland bird with mottled and streaked brown plumage, which provides camouflage in its reed bed habitat. The species has a dark streak down each side of the neck, a pale throat, and olive legs. The juvenile Nankeen Night Heron Nycticorax caledonicus is similar in appearance, but is more striped on the foreparts, with rows of white spots on the wings, and yellow legs. The Black Bittern Ixobrychus flavicollis is smaller and darker, with a streaked throat. The Australasian Bittern is partly nocturnal and cryptic but may be recognised during breeding by the male’s deep booming calls.
2. The Australasian Bittern is an endemic Australasian species whose range includes New Zealand. In Australia, it occurs in three regions: south-eastern Australia from the Queensland border to south-east South Australia, south-west Western Australia and Tasmania. These regions are inferred to support three subpopulations. Australasian Bitterns in NSW form a part of the south-eastern subpopulation and are found in the riparian and wetland areas in the east and south of the state.
3. The Australasian Bittern inhabits temperate freshwater wetlands and occasionally estuarine reedbeds. The species favours permanent shallow waters, or edges of pools and waterways, with tall, dense vegetation such as sedges, rushes and reeds on muddy or peaty substrate. Australasian Bitterns also occur in Lignum Muehlenbeckia florulenta and Canegrass Eragrostis australasica on inland wetlands.
4. The Australasian Bittern builds a platform nest of reeds and rushes just above water, in the deep cover of tall dense stands of reeds or rushes. A clutch of up to four to five eggs may be laid in spring to summer, although limited information is available. Breeding pairs of Australasian Bitterns are solitary and territorial, occupying relatively large home ranges of 40-50 ha and occurring at low densities. Generation length is estimated as five years, with low reliability (Garnett & Crowley 2000).
5. The Australasian Bittern feeds on animals in and around the margins of wetlands including: fish, crayfish, frogs, insects, snakes, lizards and occasionally small birds and mammals. Plant matter can also form part of the diet.
6. In recent decades, the Australasian Bittern is believed to have undergone a reduction in population size in NSW, based on comparative evidence from broadscale surveys. The species was reported in 32 one-degree grids in the first national bird atlas in 1977-1981, at low to high reporting rates (Blakers et al. 1984), and 30 grids in the second national bird atlas in 1998-2002, at low reporting rates (Barrett et al. 2003). Although the index of abundance (reporting rate) for the species declined by 25% in NSW between the two atlases (i.e. over 20 years or four generations), with no variation between bioregions (Barrett et al. 2007), this was not statistically significant.
7. Reduction over many years of suitable wetland habitat has potentially impacted populations of the Australasian Bittern. The Lowbidgee (confluence of the Lachlan, Murrumbidgee and Murray Rivers), which is a core habitat area for the Australasian Bittern, has shown significant declines in shorebird populations over 19 years to 2001, and waterbirds in this area (covering all functional groups, including large wading birds) also declined by over 90% (Kingsford & Thomas 2004). Over 15 years to 2007, aerial surveys have found that in eastern Australia, total numbers of waterbirds have declined by around 75%, with a decline in breeding index of roughly 90% (Kingsford & Porter 2009).
8. The number of mature individuals of the Australasian Bittern has been estimated as 2 500 nationally (Garnett & Crowley 2000; Wetlands International 2006). This estimate is assigned a low level of reliability. Birdlife International (2009) has suggested that the Australian population may be even lower than this, given the recent severe reduction in habitat and breeding grounds, and provides an estimate of not more than 1 000 mature individuals. Around a half to two-thirds of the national population is estimated to occur in NSW on the basis of geographic range and the distribution of suitable wetland habitat. Assuming as an upper limit that two-thirds of the national population occurs in NSW, and based on an estimate of between 1 000 and 2 500 mature individuals, the plausible range of the number of mature individuals in NSW is between 660 and 1 660.
9. The national extent of occurrence (EOO) for the Australasian Bittern is estimated as 1000000 km2, with high reliability, and the national area of occupancy (AOO) is estimated to be 1 200 km2, with low reliability (Garnett & Crowley 2000). Based on these figures from 2000, and considering that a half to two-thirds of the species’ distribution is thought to occur in NSW, the EOO of the species in NSW is thus estimated to be between 500 000 and 660 000 km2 and the AOO is estimated to be between 600 and 800 km2. However, as a result of loss of wetland habitat since 2000, it is likely that these figures are now lower.
10. Drainage of wetlands and alteration of natural flow regimes can result in the loss of breeding habitat and nest sites of the Australasian Bittern as well as depletion of their food supply and foraging habitat. The inland habitat of the Australasian Bittern is subject to diversion of water for irrigation, and salinisation and drainage of permanent swamps (Garnett & Crowley 2000; Olsen & Weston 2004). Major floodplain wetlands in the Murray-Darling Basin have had up to a 60% reduction in flow, and consequently 40-77% of their area has been destroyed or degraded over the past century (Kingsford & Thomas 2004; Kingsford & Porter 2009). Coastal habitats of the Australasian Bittern are under intense pressure from housing, rural-residential and other developments (Olsen & Weston 2004). In NSW, it is estimated that from 30% to as much as 90% of coastal freshwater wetlands have been destroyed, with much of the remainder fragmented, degraded and under continuing threat (NSW Scientific Committee 2004). Grazing and burning of wetland habitats may also threaten the Australasian Bittern. ‘Alteration to the natural flow regimes of rivers, streams, floodplains and wetlands’ is listed as a Key Threatening Process under the Threatened Species Conservation Act 1995.
11. Potential threats to the Australasian Bittern include predation by the European Red Fox (Vulpes vulpes) and anthropogenic climate change resulting in decreased rainfall and further alteration to natural flows. ‘Anthropogenic Climate Change’ and ‘Predation by the European Red Fox Vulpes vulpes (Linnaeus, 1758)’ are listed as Key Threatening Processes under the Threatened Species Conservation Act 1995.
12. Australasian Bittern Botaurus poiciloptilus (Wagler, 1827) is not eligible to be listed as a Critically Endangered species.
13. Australasian Bittern Botaurus poiciloptilus (Wagler, 1827) is eligible to be listed as an Endangered species as, in the opinion of the Scientific Committee, it is facing a very high risk of extinction in New South Wales in the near future as determined in accordance with the following criteria as prescribed by the Threatened Species Conservation Regulation 2010:
Clause 8
The estimated total number of mature individuals of the species is:
(b) low,
and
(d) a projected or continuing decline is observed, estimated or inferred in the key indicator:
(b) geographic distribution, habitat quality or diversity, or genetic diversity.
Dr Richard Major
Chairperson
Scientific Committee
Proposed Gazettal date: 05/11/10
Exhibition period: 05/11/10 - 21/01/11
References:
Barrett G, Silcocks A, Barry S, Cunningham R, Poulter R (2003) ‘The New Atlas of Australian Birds.’ (RAOU: Melbourne)
Barrett GW, Silcocks AF, Cunningham R, Oliver DL, Weston MA, Baker J (2007) Comparison of atlas data to determine the conservation status of bird species in New South Wales, with an emphasis on woodland-dependent species. Australian Zoologist 34, 37-77.
Blakers M, Davies SJJF, Reilly PN (1984) ‘The Atlas of Australian Birds.’ (Melbourne University Press: Melbourne)
BirdLife International (2009) Species factsheet: Botaurus poiciloptilus. Downloaded from http://www.birdlife.org on 10/11/2009
Garnett S, Crowley G (Eds) (2000) ‘The Action Plan for Australian Birds 2000.’ (Environment Australia: Canberra)
Kingsford RT, Porter JL (2009) Monitoring waterbird populations with aerial surveys – what have we learnt? Wildlife Research 36, 29–40.
Kingsford RT, Thomas RF (2004) Destruction of wetlands and waterbird populations by dams and irrigation on the Murrumbidgee River in arid Australia. Environmental Management 34, 383-396.
NSW Scientific Committee (2004) Final determination: Freshwater wetlands on coastal floodplains of the NSW North Coast, Sydney Basin and South East Corner bioregions – endangered ecological community listing. NSW DECC: Sydney (gazetted 17/12/2004).
Olsen P, Weston M (2004) The State of Australia’s Birds 2004: Water, Wetlands and Birds. Supplement to Wingspan 14(4) (accessed Feb 2010).
Wetlands International (2006) Water Bird Population Estimates 4th Edition (Wetlands International: Wageningen, The Netherlands)