Document ID: chunk:federal_register_of_legislation:F2023L00138:front:0:p8
Version: federal_register_of_legislation:F2023L00138
Segment Type: other
Provision Reference: 
Character Range: 19990–22966

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 between April and October, with a peak numbers in August (Kingsford et al. 2014). Between 2015–2018, satellite tracking demonstrated that birds move between the Murray Darling Basin and coastal South Australia, Victoria and New South Wales, so the winter influx around Melbourne is likely to be of birds which breed in the Riverina region (Bitterns in Rice 2019). The mobility of Australasian Bitterns makes the species hard to census as there is a risk of double counting birds recorded at different wetlands.

   Based on occupancy modelling of the rice fields in southern New South Wales, and in the absence of similar work in other locations, rice fields are now known to support about 39 per cent of the total global population and 59 per cent of the Australian population (Herring et al. 2019a). These agricultural wetlands are typically flooded from October-April, with Australasian Bitterns commencing nesting in December and January when there is sufficient cover and prey. Rice height is a maximum of around 1.1 m, with water depths usually 15–25 cm. The most important prey for Australasian Bitterns in rice fields is tadpoles, frogs, fish and dragonfly larvae. In "early permanent water" crops, ponding is consistent from October-April, and potential fledging of all chicks prior to harvest is the norm, however a contraction of the ponding period – "delayed permanent water" – shorter season varieties and mid-season drainage are agronomic trends likely to reduce opportunities for successful breeding.

             Rice fields are a seasonal attraction and Australasian Bitterns make widespread use of the surrounding irrigation infrastructure, such as vegetated channels and recycle dams, particularly between rice growing seasons (M. Herring, pers. comm.
             2019). 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 Australasian Bitterns, suggests a decline in the rice field population is likely (Herring et al. 2019a).

             There is very little data available but age of maturity of the Australasian Bittern is estimated to be one year and the life expectancy is estimated to be 11 years. These figures are based on figures for Eurasian Bittern (Botaurus stellaris). The generation length for the species is estimated to be 5.5 years (BirdLife International 2019).

1.7            Habitat critical to the survival of the Australasian Bittern

             As noted above, the habitat, or biophysical environment, of the Australasian Bittern varies across its range, so it is not