The thick-billed grass wren (Amytornis) are endemic species of Australia. Amytornis modestus is a sub-species of grass wren found throughout the arid regions of north-western New South Wales (NSW), northern parts of South Australia, through to southern sections of the Northern Territory. With Western thick billed grass wren (A. textilis) found in arid areas across Western Australia and parts of South Australia. A. modestus is speculated to still occur in fragmented populations in the Grey Range, Sturt National Park. The thick-billed grass wren eats mainly insects and other small invertebrates, as well as plant seeds. Both the NSW government office of environment and heritage and the Commonwealth of Australia classify the thick billed grass wren as critically endangered. However the South Australian government and the IUCN red list classifies the thick billed grass wren as ‘least concern’. It was gazetted under the Environmental Protection and Biodiversity Conservation (EPBC) Act 1999, in 2009.
Taxonomy and evolution
Most current sources refer to two subspecies. An eastern (A. modestus) and western (A. textilis) form. They are considered two species because of their differences in colouration, habitat, and distribution. Their genetic sequences are also dissimilar to classify them as separate subspecies of Thick-billed Grasswren.
Description and field identification
Dull brown underparts, noticeable white streaking on head. With white streaks continuing down the neck, throat and down to the rump. Underparts are pale fawn colour, with long, dark-brown tail. Males have distinguishably longer tales. Females also have chestnut flanks. Both sub-species are very similar in appearance. A. modestus has a paler underbelly then A. textilis. The white streaks on A. textilis are also more prominent, as well as a more upright tail and darker colourings on its throat and chest. Vocals are a combination of short high-pitched song, repeated.
Breeding occurs between July and September. Nests can usually be seen low branches in saltbush, can-grass and other similar vegetation. The nest composes of loose grass and bark in the shape of a half dome, with finer grass, fur, feathers lining the nest. One to three (usually two) eggs of various colourings of white, cream and pink; with blotches of red-brown or purplish grey. With an incubation period of two weeks. Nestling period is usually 10-12 days. Four years is the estimated generation length of the Thick-billed Grass-wren.
Usually sedentary, these illusive birds are seen running, hopping or rarely flying, between vegetative cover to remain undetected. They can also be seen foraging for food at ground level around vegetation for a wide variety of seeds, berries and macroinvertebrates. Wrens have a generalist beak type that allows them to eat a range of foods. The thick bill of A. modestus and A. textilis would allow for tougher seeds and other food niches to be accessed, compared with the smaller fairy wren species. They have a soft, high-pitched call that is often inaudible to human ears. If disturbed, individuals take refuge in any existing cover – usually vegetative or piles of old flood debris along dry sandy watercourses and even down rabbit burrows. Often seen solitarily or in pairs. Sources vary, but mating pairs maintain between five, and 20 to 40 hectare territories year-round and rarely, possibly never, band with their neighbours outside the breeding season. Family groups are sometimes seen during the post-fledgling period, while the young are still dependent on their parents.
Chenopod scrublands (consisting largely of saltbush), sandhill cane-grass and flood debris in dry, sandy watercourses. They favour the scrublands with dense chenopod bushes. These denser shrublands usually occur in lower lying areas, such as watercourses and drainage lines. Both subspecies, occurring at different distributions prefer particular species. The Eastern Thick-billed Grasswren prefers saltbush. The Western Thick-billed Grasswren; black-bush and Australian boxthorn.
The feather patterns/markings imitate their preferred habitat. This is a form of camouflage. The white streaks across the chin to the forehead and along the wings and rump, contrast with the red-brown to grey colours of the feathers.
Threats and human interactions
Because the density of humans in the areas were thick-billed grass wren occur is so low, direct human interactions are rare. Humans indirectly cause the threats. The main threat to thick-billed grass wrens is loss of habitat, through clearing, and overgrazing by hard hooved animals, such as sheep and goat, that trample the vegetation. This reduces the area and quality of the habitat that the thick-billed grass wren prefer. Particularly the larger shrubs that provide prime habitat for breeding. Habitat modification has also occurred due to rabbits. Feral species, such as foxes and cats are major threats due to predation. All threats have been compounded by droughts over the past decade.
High frequency wildfires are also a threat to the quality of habitat. Higher frequency of fires disrupts the life cycle processes of the thick-billed grass wren, as well as the plants and macroinvertebrates that they depend on. Climate change is another threat not considered in the NSW recovery plan for A. modestus. Changes in expected environmental conditions, will also challenge tolerance ranges and exacerbate the impacts of existing threats to the species.
Activities to assist this species (NSW Government)
Control vertebrate pest populations (e.g. foxes, cats and rabbits), that either prey on, or compete against this species for resources. Reduce stock intensity of, or exclude grazing in, some areas to allow regeneration of vegetation for habitat, such as food sources or nest sites. Restrict cultivation around suitable ground habitat. Retain understorey shrubs and allow them to complete their life cycle (i.e., seed set, germination, establishment, growth to maturity). Prevent clearing of habitat, such as nesting sites and food sources.
The Thick-billed Grass-wren is a poor flyer, and because of this has poor dispersal capability, and is highly susceptible to population fragmentation. The species ability to recover in NSW is currently considered low given the lack of any confirmed records of the Thick-billed Grasswren in NSW for almost half a century. The species decline appears to be attributed to a combination of factors relative to a broad scale change in vegetation structure that continues today. Active management using rational comprehensive approaches would be useful in future to ensure species viability.
- †A. m. modestus – (North, 1902): Now extinct. Formerly found in Northern Territory (central Australia)
- A. m. indulkanna – (Mathews, 1916): Found in Northern Territory and South Australia (central Australia)
- A. m. raglessi – Black, 2011: Found in Flinders Ranges in South Australia (central Australia)
- A. m. curnamona – Black, 2011: Found in Lake Frome Basin in South Australia (central Australia)
- A. m. cowarie – Black, 2016: Found in Sturt Stony Desert in South Australia (central Australia)
- A. m. obscurior – (Mathews, 1923): Found in New South Wales (central Australia)
- †A. m. inexpectatus – (Mathews, 1912): Now extinct. Formerly found in New South Wales (central Australia)
- BirdLife International (2009). "Amytornis textilis". IUCN Red List of Threatened Species. Version 2008. International Union for Conservation of Nature. Retrieved 21 February 2012.
- "Amytornis modestus — Thick-billed Grasswren (eastern)". Department of Sustainability, Environment, Water, Population and Communities. Australian Government. Retrieved 21 February 2012.
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- IOC v.6.3
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McAllan, IAW (2000). On some New South Wales records of the Grey Grasswren and the Thickbilled Grasswren. Australian Bird Watcher 18, 244-246. NPWS (2002). Thick-billed Grasswren (eastern subspecies) Amytornis textilis modestus (North, 1902) Recovery Plan. NSW National Parks and Wildlife Service, Hurstville NSW.
Office of Environment and Heritage (2016). Thick-billed Grasswren (eastern subspecies) – profile. Retrieved from http://www.environment.nsw.gov.au/threatenedspeciesapp/profile.aspx?id=10049
Pizzey, G., & Knight, F. (2012). The field guide to the birds of Australia. Harper Collins Publishers: Australia.
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