The brown thornbill (Acanthiza pusilla) is a passerine bird usually found in eastern and south-eastern Australia, including Tasmania.

They are found in dry forests with dense undergrowth, rainforests, shrublands, coastal dune thickets, and in rushes and bracken along rivers and creeks.

[3] Their calls vary from a mellow baritone pee-orr, high whistles with rapid cascading trills, to many squeaks and churrs.

[8][10] Adult brown thornbills are able to mimic the alarm calls of other birds such as the New Holland honeyeater that warn of a raptor approaching, which deters other predators such as pied currawongs from attacking their nests.

[11] The brown thornbill is mainly an insectivore, including spiders, beetles, lerp insects, ants and grasshoppers in its diet.

[12][3] It also forms mixed-species feeding flocks with the yellow-rumped thornbill (Acanthiza chrysorrhoa), the white-browed scrubwren (Sericornis frontalis), the speckled warbler (Pyrrholaemus sagittatus), and the eastern yellow robin (Eopsaltria australis).

Their nests are dome-shaped with a hooded side-entrance and built out of grasses, bark shreds, moss and feathers or plant down, lightly bound with spider webs, and usually set low in the undergrowth among ferns or tussocks.

[1] The King Island brown thornbill (A. p. archibaldi) is considered to be critically endangered and most likely to go extinct, with very few sightings for many decades.

[14] However, recent surveys indicate that there may be between 20 and 50 individuals of the subspecies living on the island and consequently a recovery program was expected to be formulated in late 2019.