Slender-billed Thornbill Acanthiza iredalei


Justification of Red List category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
Garnett and Crowley (2000) estimated the population size as follows: approximately 100,000 individuals of subspecies liredalei, approximately 10,000 individuals of subspecies rosinae and approximately 24,000 individuals of subspecies hedleyi, giving an overall total of 134,000 individuals. (Garnett and Crowley 2000).

Trend justification
This population is suspected to be in decline owing to ongoing habitat destruction caused by over-grazing by introduced rabbits and sheep (Garnett and Crowley. 2000).

Distribution and population

This species is endemic to South Australia and Western Australia, Australia. The nominate subspecies is widespread through the arid and semi-arid regions of both states. Although there has been a recent contraction in the east of its range, its extensive range outside the agricultural zone appears to be unaffected. Subspecies rosinae has a very restricted distribution, with three principle populations along the northern shore of the Gulf of St Vincent, from St Kilda to Ardrossan. Subspecies hedleyi has a fragmented distribution, primarily across Big and Little Deserts, Victoria, and Ninety Mile Plain, South Australia.


Much of the chenopod shrublands used by iredalei are degraded by grazing sheep and rabbits. However, a reduction in grazing on the Nullarbor of both sheep and rabbits may be allowing some recovery of habitat. Subspecies rosinae is threatened by recreational and industrial use of saltflats, and future coastal developments. Clearance for agriculture has affected hedleyi in South Australia, particularly in the 1980s, but has now largely ceased, though large-scale fires are frequent and may be affecting abundances. Its populations in the Big and Little Deserts appear fragmented despite large areas of protected, suitable habitat (Garnett and Crowley 2000).


Text account compilers
Ekstrom, J., Butchart, S., Harding, M., Fisher, S.

Recommended citation
BirdLife International (2023) Species factsheet: Acanthiza iredalei. Downloaded from on 30/09/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from on 30/09/2023.