The Brown Thornbill - Acanthiza pusilla - is a small (10 to 11 centimetres) brown bird found in several slightly different coloured versions. Head and upper parts are rich olive-brown to brownish grey; the face is flecked olive-grey to pale-grey with russet to white scallops on the forehead. Bill is black. Throat and breast streaked grey-black. Wings are dark grey-brown with paler edges. Tail is dark grey-brown. Underparts are cream to white grading to olive-brown, grey or yellowish on the flanks and undertail.
Male and female are similar. Immature birds are duller in colour and more lightly streaked on the breast. Adult plumage is achieved after the first body moult in autumn.
This species displays considerable variation and there are seven versions; five are olive-brown with russet patches above the eyes; one grey species with white patch above the eyes and one intermediate species (on Kangaroo Island, SA).
Brown Thornbills are sedentary birds in a variety of habitats, from rainforest to arid mulga woodlands. Found along a broad strip along the eastern coast from Proserpine (Qld) south to the Yorke Peninsula and Kangaroo Island (SA) including all of Victoria, except the driest north-west; also in Tasmania. They are not spread evenly across habitats but are found in pockets around taller close shrubs one to five metres high. Established pairs hold territories of a quarter to five hectares all year round, advertising them with regular singing. Young forage with their parents in family parties in months after breeding but are expelled from the territory in autumn and early winter. Many of this new generation then die in unfamiliar habitats; those that find their own territory may live for 10 years or longer.
Brown Thornbills spend most of the day foraging in the mid and upper shrubs. They hop among foliage and along twigs working through each bush before flitting jerkily to the next. Insects, and sometime seeds, are picked from leaves and under bark. Sometimes nectar is taken from flowers.
Breed from June to December; in colder area breeding may be delayed until September. The nest is an untidy dome with a hooded side entrance near the top made of bark, coarse grasses and ferns bound with cobweb and lined with finer grasses, fur and feathers. Built by the female in low branches, shrubs, fern thickets or under tussocks on the edge of banks. There are two or usually three eggs, white to pale flesh coloured, liberally sprinkled and blotched with red-brown, mostly at the large end; oblong-oval, 16 millimetres by 12 millimetres. Incubation takes 17 to 21 days by the female alone, the male remains nearby to defend the territory. Young fledge in about 15 days.