• Summary
• Text account
• Data table and detailed info
• Distribution map
• Reference and further resources

Justification of Red List category
This species has a small population that is declining primarily owing to the degradation of its wetland habitats, with less than 1,000 mature individuals in the largest subpopulation if, precautionarily, the plausible lower bound is followed. For these reasons it is assessed as Vulnerable.

Population justification
In 2009–2010, there were estimated to be 3–16 Australasian Bitterns on natural wetlands in Queensland, 82–162 in New South Wales, 86–248 in Victoria, 12–100 in Tasmania, 26–116 in South Australia and 38–154 in Western Australia (Garnett et al. 2011). The Western Australia population is now considered to comprise fewer than 100 individuals (WA Recovery Team unpublished, in Herring et al. 2021), while Tasmania is now thought to support 20–80 birds (E. Znidersic unpublished, in Herring et al. 2021). Previously overlooked, c.500–1,000 are estimated to use rice fields in the Riverina, depending on the area sown, while the Barmah-Millewa wetland complex is considered Australia’s most important site, with up to 73 booming males (Herring et al. 2019). In total c.950 of the eastern mainland birds are thought to occur in the Murray–Darling Basin, of which approximately half move to the coast in winter where another 200 live year-round (M. Herring unpublished, in Herring et al. 2021). Overall the Australian population is estimated to number 750-1,800 mature individuals, with a best estimate of 1,300. In New Zealand, the population was estimated at 580–725 birds in 2000 (Heather and Robertson 2000) which is now thought to be too high given recent declines at key wetlands including Whangamarino (O’Donnell and Robertson 2016). Additionally, recent research suggests that individuals may have been double counted in the past due to birds travelling long distances to find different wetland habitats (Ornithological Society of New Zealand 2019). The most recent estimate of the New Zealand population is 125 pairs (Robertson et al. 2021), equivalent to 250 mature individuals. Overall the global population is estimated at c.1,600 mature individuals and best placed in the band 1,000-2,499 mature individuals, equating to 1,500-3,749 individuals in total and rounded here to 1,500-4,000.

Trend justification
Based on published data, declines in the Australian population have been occurring for decades and have been associated with loss of key breeding habitats (Commonwealth of Australia 2019). In the past, the reporting rate in national Atlas surveys decreased from being recorded in 260 10-minute grid squares in 1977-1981, to 142 squares in 1998-2003, and 61 in 2003-2008 (Birds Australia 2010, Garnett et al. 2011). The reporting rate declined by >90% in Tasmania and Western Australia, and by 63% in the Riverina. The long-term rate of decline was overall estimated to exceed 20% over two generations, and the area of occupancy in Australia is thought to have declined by 70% between 1977 and 2008 (Commonwealth of Australia 2019). However, currently there is no evidence for a population decline exceeding 20% in two generations, and although a continuing decline is inferred, the rate is not known (Herring et al. 2021). Riverina's rice fields support c.60% of the national population and declines are thought to be continuing here given reduced water allocations associated with drought, the trend towards cotton and other alternative irrigation water uses that do not support bitterns, and a shift away from “early permanent water” rice crops, which are favoured by bitterns (Commonwealth of Australia 2019). In New Zealand (c.15% of the global population) the species is thought to be rapidly declining at a rate equivalent to 50-70% within three generations (Robertson et al. 2021).

Botaurus poiciloptilus occurs in the wetlands of southern Australia, New Zealand and New Caledonia. In Australia they occur in south-eastern mainland Australia from south-east Queensland to south-eastern South Australia as well as Tasmania (Marchant and Higgins 1990). In south-western Australia, they occur in nine wetland complexes from Perth to Esperance (DBCA 2018). Previously, they occurred between Moora and Cape Arid. Half the wetlands on the Swan Coastal Plain that supported the species in 1980 are no longer suitable (Pickering and Gole 2008). In south-eastern Australia, including Tasmania, fewer than 30 natural wetlands are now thought to support breeding in most years, or support more than one booming male, with the number substantially lower in drought years. A further 50–1,130 km² of suitable breeding habitat occurs in rice fields of the New South Wales Riverina, especially those receiving early permanent water, with the area sown to rice varying by over an order of magnitude according to water availability. Satellite tagged birds disperse from the Riverina to coastal wetlands in South Australia, Victoria and New South Wales (Herring et al. 2019). Australasian Bitterns also occur in New Zealand (Marchant and Higgins 1990) and have occasionally been recorded in New Caledonia (Spaggiari et al. 2007) although these records are unconfirmed and the species may be extinct here (G. Dutson in litt. 2020, A. Silcocks in litt. 2022).

Australasian Bitterns occupy shallow, vegetated freshwater or brackish swamps, favouring those dominated by sedges, rushes and/or reeds from 0.5 – 3.5 metres in height (Marchant and Higgins 1990), as well as rice fields (Herring et al. 2019), where they feed on fish, eels, frogs, freshwater crayfish and aquatic insects (Heather and Robertson 1997, Menkhorst 2012). They are also known to prey on small mammals (Gitsham 2018), and on occasion they can feed away from waterbodies such as in grassland or crop stubble, particularly during rodent plagues (A. Silcocks unpublished, in Herring et al. 2021). The population seems to increase rapidly in good years and decline rapidly in poor ones (S. Garnett in litt. 2003). The species can disperse widely, including seasonal movements of 400-600 km from inland rice fields to coastal wetlands; during periods of drought; and to ephemeral wetlands during and after periods of rainfall (Garnett et al. 2011, Bitterns in Rice Project 2020).

Historically, Australasian Bitterns have lost habitat because of drainage of permanent and ephemeral swamps for agriculture or urban development, the diversion of water for irrigation, and reduced water inflows and salinisation (Kingsford 2000). However, most of the key natural breeding wetlands are now protected, and others are being restored. The greatest threat is an increased frequency and intensity of droughts as is predicted (Evans et al. 2017), which is likely to exacerbate the already extreme fluctuations in wetland habitat availability combined with seasonal shifts in rainfall. In dry periods there is also less water available for rice farming, which supports around 60% of the Australian population, and greater pressure on rice farmers to use water-saving agronomy or switch to alternative crops like cotton (Herring et al. 2019). Dry periods also make wetlands, including peat swamps, susceptible to fire, which may take many years to regain their habitat suitability (DBCA 2018). Foxes Vulpes vulpes and possibly Cats Felis catus may also have population-level impacts by preying on juveniles or adults. Invasive herbivores such as Feral Pigs Sus scrofa, horses, Feral Goats Capra hircus and deer degrade wetlands important for Australasian Bittern by digging up wetland edges, removing vegetation cover or trampling wetland vegetation (DBCA 2018). The New Zealand population continues to be threatened by habitat loss and degradation, as well as predation (O'Donnell and Robertson 2016). Nests have been reported to be abandoned following visits by people, implying that the species is sensitive to disturbance (O’Donnell 2011). Shooting and flying into powerlines are additional contributory causes (B. D. Bell in litt. 1994), but hunting pressure is very low (N. Barré in litt. 2003).

Conservation Actions Underway
In Australia, Bool Lagoon and Lake Muir are managed specifically for the species (Garnett and Crowley 2000). BirdLife Australia started a Bittern Project in 2007. Most natural breeding wetlands are conserved or restoration has commenced. The species is listed as threatened under appropriate legislation, including as a National Threatened Species Strategy priority. Major conservation program run with rice farmers. Environmental water delivery at key sites incorporates Australasian Bittern requirements. WA Recovery Team established.

Conservation Actions Proposed
Complete field surveys to determine its current global distribution, status and key sites for conservation (N Barré in litt. 1999, C. O'Donnell in litt. 1999, Garnett and Crowley 2000). Develop methods for assessing population trends (Garnett and Crowley 2000). In Australia, establish a national monitoring program, incorporating known sites and strategically targeting potential habitat on private property. Determine ecological requirements at drought refuge sites.  Assess population impacts of fire and predators, particularly foxes and cats. Map known and potential habitat and determine habitat management guidelines for increasing Australasian Bittern density and breeding success. Compare genetic diversity of western, eastern and Tasmanian populations. Rehabilitate selected former breeding wetlands (Garnett et al. 2011). Encourage cultivation of bittern-friendly rice in the Riverina. Ensure water made available for known Australasian Bittern drought refuges. In partnership with land managers, restore Australasian Bittern habitat at selected sites across its range. Manage water levels, water quality, predators, fire and vegetation structure to maximise density and breeding success. In New Zealand, ensure that wetland habitats are protected and mammalian predators are managed (O'Donnell and Robertson 2016). Develop a national recovery plan for New Zealand (O'Donnell and Robertson 2016).

71 cm. Large, brown bittern. Brown-and-buff mottling. Thick neck. Partly nocturnal. Voice Deep booming calls during breeding.

Text account compilers
Vine, J.

Contributors
Barré, N., Bell, B.D., Dutson, G., Garnett, S., Jaensch, R., O'Connor, J., O'Donnell, C. & Silcocks, A.

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Recommended citation
BirdLife International (2024) Species factsheet: Australasian Bittern Botaurus poiciloptilus. Downloaded from https://datazone.birdlife.org/species/factsheet/australasian-bittern-botaurus-poiciloptilus on 27/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org/species/search on 27/12/2024.