Document ID: chunk:federal_register_of_legislation:F2023L00138:front:0:p7
Version: federal_register_of_legislation:F2023L00138
Segment Type: other
Provision Reference: 
Character Range: 17266–20264

The area of occupancy of the Australasian Bittern in Australia is thought to have declined by 70 per cent between 1977 and 2008. These declines are considered to have led to a comparable decline in the size of the adult population.

             Population estimates are largely based on skewed citizen science records and there is a degree of uncertainty of population trends across the range of the Australasian Bittern. Further complicating the matter is the level of mis-identification inherent in Australasian Bittern records. Immature Nankeen Night Herons (Nycticorax caledonicus) and Australian Little Bittern (Ixobrychus dubius) can be mistaken for Australasian Bittern by inexperienced observers.

         1.6            Ecology

   The knowledge base on Australasian Bittern is relatively weak and based on a small number of localised studies or observations, consistent with the secretive nature of the species and difficulty for researchers in accessing its preferred dense, swampy
   habitats. Concerted programs of observation by BirdLife Australia and some government agencies in the past two decades have improved knowledge of occurrence and numbers (Herring et al. 2016, 2019)

   The Australasian Bittern occurs solitarily, in pairs or dispersed aggregations of up to 34 birds (M. Herring, pers. comm. 2019). It is likely to be sedentary in permanent habitats, but regular post-breeding dispersal of between 400–600 km from inland rice fields to coastal wetlands has been recorded in south-eastern Australia (Herring et al. 2016; Bitterns in Rice Project, 2019).

   The Australasian Bittern breeds from October to February in solitary pairs, or polygamously with up to three nesting females per booming male (Bitterns in Rice Project 2016). The species nests in densely vegetated freshwater wetlands, building its nests within dense cover over shallow water placed about 30 cm above the water level (Marchant and Higgins 1990). In rushland, it may avoid breeding in the densest areas (Marchant and Higgins 1990), and in reedbeds may prefer smaller patches, rather than continuous stands (M. Herring, pers. comm. 2019). If population densities are high, it may resort to more open wetlands for nesting, such as in stunted Acacia swamps (Marchant and Higgins 1990). Clutch size is usually four or five, but ranges from three to six (Serventy and Whittell 1976; Marchant and Higgins 1990).

   The Australasian Bittern appears to be capable of moving between habitats as suitability changes. It can occur in high densities in temporary or infrequently filled wetlands during exceptionally wet years, and will also use ephemeral wetlands when moving from areas that are drying out (Garnett et al. 2011). Monitoring data provides evidence of seasonal movements and that they respond to favourable conditions at key wetlands. Analysis of data collected at regularly monitored wetlands in the Melbourne area (monthly surveys) shows an influx of birds during the non-breeding season