The dusky grasswren (Amytornis purnelli) is a species of small passerine bird in the family Maluridae. The species is endemic to Australia but is limited to inland areas of the Northern Territory, Western Australia and South Australia.
Description
The dusky grasswren is superficially similar in appearance to a number of the other grasswrens in the genus Amytornis, but is the only member of this genus to exclusively inhabit rocky ranges across Central Australia. It has a predominantly brown base colour to its plumage with fine pale streaking over the head breast and mantle. The brown of the plumage is a richer chestnut on the back and pale to almost buff on the belly and vent. The most obvious difference between sexes is the bright rusty-brown spot on the fore-flanks of the adult female and all birds exhibit the characteristic cocked tail posture common to all malurids.
Field identification
The species is easily identified within its core range by its distinctive vocalisations (see further information below). The mouse-like, strongly terrestrial, habits of the species can make it difficult to observe clearly within its preferred habitat of boulder-strewn rocky hillsides covered with spinifex grasses. But when observed well, the cocked tail, overall brown colouration and strongly terrestrial habits should be sufficient to achieve a positive identification.
Similar species
The dusky grasswren is similar in shape and form to most of the other species in Amytornis, but is relatively unmistakable if seen well in the field. It probably doesn't overlap in its use of habitat with any other species of grasswren and also differs in its vocalisations. The species most likely to cause confusion are the Kalkadoon grasswren (Amytornis ballarae), with which it was considered conspecific until 1999, and the thick-billed grasswren (Amytornis modestus). The former only occurs well to the northeast of the currently-known distribution of dusky grasswren; the latter is considered extinct in the Northern Territory and only occurs to the south and east of the known distribution of dusky grasswren in South Australia. Dusky grasswren might also be confused with striated grasswren (A. striatus) but is not currently-known to occur in the sand plain and dunefield habitat of this species. Striated grasswren is also so boldly marked by comparison to dusky grasswren that it should be unlikely to cause any confusion in the field. Fairywrens within family Maluridae overlap with dusky grasswren in both their distribution and use of habitat including: Splendid Fairywren (Malurus splendens); Purple-backed Fairywren (M. assimilis); and White-winged Fairywren (M. leucopterus). Although these species may display a superficial resemblance to the dusky grasswren in their contact calls, their posture and manner of moving, and their general size and shape, if properly seen and heard in full song they are easy to separate. The fairywren species are all a uniform grey colour (except for breeding males which will sport brightly-coloured alternate plumage) with varying amounts of blue visible on adult's tails.
Vocalisations
This species most often reveals itself initially with a sharp, metallic tutting in alarm or a drawn-out seeeep contact call. The vocal repertoire of the dusky grasswren is varied and includes trills, high-pitched whistles, rapid twittering, and high-speed warbling. Much of the richness in the calls is difficult for humans to detect due to the extreme speed with which they are performed. When recordings are slowed however, the extensive lexicon used by the species becomes more obvious.
Geographic variations
Some geographic variations have been noted but further research is required before a determination can be made as to whether these variations are sufficient to warrant taxonomic recognition. There are vocal differences (more stentorian and staccato in the north of their range) as well as plumage differences (birds are observed to be paler in the north of their range and richer rufous-brown in the south).
Ecology
Habitat
Across their entire range, dusky grasswrens adhere to the same habitat type: rocky ranges and outcrops, often preferring tumbled talus or scree, and with areas of thick, long-unburnt spinifex grasses in the genus Triodia. They are not known to use sandplain habitat adjacent to rocky areas which may be one factor giving rise to the patchiness of their occurrence across Central Australia.
