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Brown Thornbill (Acanthiza pusilla)

The Brown Thornbill is a small, mostly sedentary passerine of dense shrub layers in forests, woodlands, heathlands, rainforests, gardens and vegetation along watercourses. The widespread species is not nationally listed, but the isolated King Island subspecies, Acanthiza pusilla magnirostris, also referred to as A. p. archibaldi, is listed as Endangered under the EPBC Act and has an extremely small, poorly known population, so clearing of dense understorey on King Island requires particular care.

Conservation status

Commonwealth, EPBC Act 1999: Endangered.

Tasmania, Threatened Species Protection Act 1995: Endangered.

Breeding season

The seasonal pattern below combines the best available information for Brown Thornbills across the range. Timing for the King Island subspecies is poorly known, so the mainland Tasmanian breeding period is used as the closest guide.

The species is present year round in suitable habitat because adults are generally sedentary. Seasonal values are indicative and should not replace site-based searches before clearing.

  • Breeding: Breeding is reported from July to January for the species, and August to January for mainland Tasmania. King Island timing is unknown.
  • Young: Nestlings remain in the nest for about 16 days. Dependent young may remain with parents until early autumn in the general species account.
  • Movement and dispersal: Adults are generally sedentary and defend permanent territories. Some dispersal of young birds may occur in winter, but movement of the King Island subspecies is unknown.
  • Highest clearing risk: Clearing and disturbance are most likely to harm the species during the breeding period, when nests and dependent young occur in low shrubs, sedges, ferns and grass clumps. Apply the higher risk period from August to January in Tasmania, while recognising that the King Island timing is unknown.

Identification

Brown Thornbills are small, active birds that usually remain within dense shrubs and low trees. Positive identification can be difficult because they move constantly and resemble other thornbills and scrubwrens.

Adults have olive-brown to grey-brown upperparts, a warm russet or reddish-brown forehead with pale scalloping, a reddish-brown rump, a grey-brown tail with a dark band and pale tip, and off-white underparts streaked with blackish markings on the chin, throat and breast.

The eye is dark red. Sexes appear similar. Young birds resemble adults but generally have a duller eye.

Length: 9 to 10 cm generally; 9 to 11.5 cm reported for the King Island subspecies.

Weight: About 7 g for the King Island subspecies.

Identification: Calls and behaviour

The call includes rich musical warbles, whistles, trills, deep notes, harsh scolding and mimicry. Birds may respond to an imitation of their call.

Brown Thornbills are usually encountered singly, in pairs or in small family groups. They actively forage through shrubs and low trees and rarely remain still for a clear view.

  • Use call and visual searches in suitable dense shrubby habitat.
  • Treat a brief visual record as provisional where similar thornbills or White-browed Scrubwrens are present.
  • For the King Island subspecies, use mist-netting after area searches or transects have located thornbills, because the longer bill may need to be examined in the hand.

Identification: Similar species and field signs

The King Island subspecies is distinguished principally by its noticeably longer bill than Brown Thornbills on mainland Australia and mainland Tasmania. Other potential confusion species include the Inland Thornbill, Tasmanian Thornbill, other thornbills and the White-browed Scrubwren.

A small domed or oval nest with a partly hooded entrance, made from grasses, bark and other material and lined with feathers, fur or soft plant down, is a useful breeding sign. Nests are usually low in prickly bushes, grass clumps, sedges or ferns.

  • Do not rely on tracks, scats, sloughed skins or eyeshine for detection. These are not described as useful field signs for this species in the supplied sources.
  • Record the bill profile, forehead colour, breast streaking, eye colour, call and exact vegetation structure in field notes and photographs where possible.

Distribution: National range

The Brown Thornbill occurs in eastern and south-eastern Australia, including Tasmania. The supplied documents do not assign the species to named Australian bioregions, so bioregion-level conclusions should be made from current state or regional mapping for the project area.

The species is widespread from south-eastern Queensland through New South Wales and Victoria to the Mt Lofty Ranges in South Australia, and occurs throughout Tasmania except the south-west. It is mainly associated with temperate and subtropical regions, from the coast to about 1200 m.

Records from the supplied survey material also include Tasmania. The widespread species is described as common in suitable habitat, but the conservation status of the isolated King Island population must not be applied to every Brown Thornbill record in Australia.

States and territories indicated: Queensland, New South Wales, Victoria, South Australia and Tasmania.

Elevation: From the coast to about 1200 m.

Named bioregions: Not specified in the supplied documents.

Distribution: King Island population

The Endangered taxon is endemic to King Island in Bass Strait. Its last known sites in the supplied profile were Pegarah State Forest in central King Island and Loorana in western King Island, about 17 km apart.

The subspecies has been recorded in Pegarah State Forest and Loorana, but current population locations are not known. Extensive searches in 2003 to 2004 did not record it, following the observation of two birds in Pegarah State Forest in March 2002.

Extent of occurrence: Suggested at 5 km², but this estimate is of low reliability.

Confirmed records: Four birds collected in 1902, one in 1968, two mist-netted in 1971 and two observed in 2002.

Theoretical maximum breeding population: Estimated at 50 breeding birds, with low confidence.

Population trend: Probably declined and is now extremely rare; the population has always apparently been small.

Habitat: Habitat types

Brown Thornbills need dense, continuous shrub cover within or beside woodland and forest. Habitat suitability depends more on the understorey and ground-level cover than on the presence of large trees alone.

The species occurs in dense shrubby habitat in wet and dry forest, woodland, shrubland, heathland and rainforest. It also uses vegetation along watercourses, parks, gardens and nature strips, especially where these are close to remnant vegetation.

On King Island, the subspecies has been recorded in dense mixed scrub, wet Tasmanian Blue Gum forest and gardens surrounded by farmland.

Habitat: Structure and site features

Important structure includes a dense shrub layer, saplings, prickly bushes, sedges, ferns, bracken and low trees. King Island examples included paperbark, dogwood, goldenwood, wattles, banksia, tea-tree, heath and bracken understorey.

A King Island record came from dense mixed scrub about 10 m tall. Another record came from a garden with African Boxthorn hedges and small paperbark thickets.

Reported scrub height: About 10 m at one King Island capture site.

Pegarah State Forest: About 1100 ha, including approximately 90 ha of the dense mixed scrub described in the historical record.

  • Retain dense understorey, shrub clumps, low prickly vegetation, sedges, ferns and bracken during site planning and clearing.
  • Check apparently isolated shrub patches, gardens and roadside vegetation where they connect to larger remnants or provide dense cover.

Habitat: Hydrology and fire context

On King Island, thickets associated with swamps are potential habitat. Drainage, stock trampling, uncontrolled fire and degradation of native vegetation can reduce its suitability.

The supplied documents do not provide a species-specific moisture threshold, fire interval or minimum patch size.

  • Avoid unnecessary drainage or alteration of swamp margins supporting dense thickets.
  • Assess fire effects on the understorey, not only canopy condition, because fire that removes dense shrub cover can reduce habitat suitability.

Foraging habitat: Diet

Brown Thornbills feed mainly on small invertebrates in dense vegetation. They forage at several levels but concentrate activity in understorey shrubs and low trees.

The diet is mostly insects, with occasional seeds, nectar and fruit. The food items of the King Island subspecies have not been documented directly, so mainland Tasmanian Brown Thornbill information is the best available guide.

Likely foraging resources include insects and other small invertebrates associated with bark, twigs, branches, saplings, shrubs, leaf litter and dense foliage.

Foraging habitat: Foraging method and locations

Birds forage mainly in pairs, actively moving through vegetation and sometimes joining mixed flocks outside the breeding season. On mainland Tasmania, recorded foraging occurred in the canopy, on twigs and branches, on trunks, in saplings and in low shrubs.

The King Island subspecies may probe bark with its relatively long bill. No species-specific foraging range has been established.

Mainland Tasmanian foraging observations: 20% in the canopy, 25% on twigs or branches, 14% on trunks, 25% in saplings and 16% in low shrubs.

Ground foraging: None recorded in the cited mainland Tasmanian observations.

Territory size: 0.25 to 3.5 ha reported for mainland populations; the King Island territory size is unknown.

  • Retain a connected mix of low shrubs, saplings, small trees, rough bark, fallen material and ground cover where possible.
  • Inspect dense understorey and low tree foliage rather than searching only the canopy.
  • Treat all-purpose territories as potentially year-round habitat because Brown Thornbills are generally sedentary.

Breeding habitat: Breeding biology

Breeding Brown Thornbills hold territories throughout the year and place their nests low in dense vegetation. The King Island subspecies has very limited direct breeding information, so most breeding details come from mainland Tasmanian Brown Thornbills.

The female builds a small domed or oval nest with a partly hooded entrance. It is made from grasses, bark and other plant material and lined with feathers, fur or soft plant down. The female incubates, and both parents feed the young.

The nest is generally low down in prickly bushes, grass clumps, sedges or ferns. Fan-tailed Cuckoos have been recorded laying in Brown Thornbill nests.

Breeding season: July to January for the broader species account; August to January for mainland Tasmania. The King Island timing is unknown.

Clutch size: Two to four eggs, usually three.

Incubation: About 19 days.

Time in nest: About 16 days.

Age at maturity: Unknown.

Lifespan: Unknown.

  • Search dense low shrubs, sedges, ferns, grass clumps and prickly vegetation for nests before clearing.
  • If a nest is found, stop work in the immediate area and apply the project's fauna handling and exclusion procedures.

Breeding habitat: Breeding habitat

Breeding habitat is dense, low vegetation that provides concealed nest sites and nearby foraging cover. Permanent territories are used for both feeding and breeding.

For the King Island subspecies, suitable breeding habitat may include dense mixed scrub, tea-tree thickets, wet forest understorey and dense thickets near swamps.

  • Prioritise pre-clearing nest searches from July to January, with particular attention to August to January in Tasmania.
  • Retain connected shrub cover around known or potential nest sites and avoid unnecessary disturbance during the breeding period.

Sheltering habitat: Cover requirements

Brown Thornbills shelter and move through dense shrubs, saplings, ferns, sedges and other low vegetation. They do not use a documented den, burrow or tree hollow.

The species uses dense understorey for concealment, resting and movement. Suitable cover includes low shrubs, tea-tree thickets, paperbarks, wattles, banksia, heath, bracken, sedges and dense garden hedges.

The King Island subspecies has been associated with dense mixed scrub, wet forest understorey and small paperbark thickets. Its nest is normally close to the ground, in or below low vegetation.

Known refuge types: Dense shrubs, saplings, ferns, sedges, bracken, tea-tree thickets, paperbark thickets and dense hedges.

Nest height: Usually low down, on or near the ground.

Hollows, burrows and dens: No use of tree hollows, burrows or dens is described in the supplied documents.

  • Retain dense shrub patches and low ground cover during vegetation removal.
  • Check the underside and interior of dense shrubs, grass clumps, sedges and ferns before machinery enters the work area.
  • Do not assume a patch is unoccupied because no bird is visible during a short inspection.

Sheltering habitat: Movement and territory

Brown Thornbills are generally sedentary and defend permanent territories. Young birds may disperse in winter, but the movement pattern of the King Island subspecies is not known.

Territories of mainland populations range from 0.25 to 3.5 ha. The King Island territory size has not been measured.

  • Maintain links between dense vegetation patches where practicable, particularly on King Island where habitat is fragmented.
  • Consider nearby dense vegetation as potential refuge and release habitat if a bird is captured during authorised fauna handling.

Threats: Clearing and fragmentation

The main risk to Brown Thornbills on development sites is loss or simplification of dense understorey. Risks are most serious for the Endangered King Island subspecies because its population is extremely small and its current locations are uncertain.

Extensive clearing and degradation of native vegetation are identified as likely major causes of decline for the King Island subspecies. Fragmentation can leave small, isolated patches that no longer support connected territories or movement by young birds.

Clearing of shrubs, saplings, ferns, sedges, tea-tree and swamp thickets can remove both feeding cover and low nesting sites, even where mature trees are retained.

  • Map and retain dense understorey, not just canopy trees, during design and construction planning.
  • Avoid clearing habitat corridors between remaining vegetation patches on King Island.
  • Stage clearing so retained vegetation remains connected and protected from machinery, spoil, vehicle access and edge damage.

Threats: Fire and hydrology

Uncontrolled fire can reduce or remove the dense understorey on which the species depends. The small King Island population is considered particularly susceptible to uncontrolled bushfire.

Drainage of swamps and trampling by stock can degrade thickets that may support the King Island subspecies.

  • Prevent escaped construction fires and manage hot works near dense shrub habitat.
  • Avoid drainage, sediment deposition and altered water flows into swamp-associated thickets.
  • Control vehicle and stock access where trampling could damage dense understorey.

Threats: Predators, competitors and disease

Domestic cats are known to prey on Brown Thornbills in suburban areas. Foxes and cats are relevant risks where nests are low to the ground.

Competition with Tasmanian Thornbills has been proposed for the King Island subspecies, but the evidence is uncertain. Ticks have also been suggested as a possible threat, without direct evidence.

  • Keep domestic animals away from retained habitat and construction compounds.
  • Manage food waste and shelter around compounds to avoid attracting cats or foxes.
  • Do not treat competition or ticks as confirmed causes at a site without supporting evidence.

Threats: Roads and construction disturbance

The supplied documents do not identify a species-specific road mortality rate or road avoidance distance. Roads and construction can still remove or divide dense habitat and increase vehicle access, noise, edge exposure and predator access.

Active nests and dependent young are most exposed during the breeding season. The species is small, vocal and constantly moving, so absence during a short site inspection is not reliable evidence of absence.

Recommended initial King Island survey effort: Area searches or line transects: 10 hours over 5 days at sites smaller than 50 ha.

Follow-up identification effort: Mist-netting: 6 hours over 3 days where thornbills have been located by searches or transects.

  • Use targeted area searches and transects in suitable habitat before clearing.
  • For the King Island subspecies, follow positive searches with mist-netting where identification requires examination of the bill.
  • Apply project exclusion zones and fauna spotter procedures around any active nest or dependent young.

Survey methodology: 1. Area searches and transects

The nationally listed taxon is the King Island Brown Thornbill, Acanthiza pusilla magnirostris, also recorded under the former name Acanthiza pusilla archibaldi. It is an Endangered, very small and poorly known subspecies, so surveys must focus on dense understorey and must allow time for confirmation in the hand.

Survey suitable habitat using land-based area searches and line transects before works begin.

Target dense mixed scrub, wet forest with a dense understorey, tea-tree thickets, woodland and swamp thickets, including retained vegetation corridors and habitat near Pegarah State Forest and Loorana where relevant.

For sites smaller than 50 hectares, use at least 10 hours of area searches or line transects over 5 days.

Search slowly through dense vegetation and record all thornbills, because the subspecies is small, rarely stops moving and may be difficult to distinguish by sight.

Recommended effort: 10 hours over 5 days for area searches or line transects at sites smaller than 50 hectares

Survey methodology: 2. Mist-netting for confirmation

Use mist-netting after area searches or transects have located thornbills in suitable habitat.

Apply at least 6 hours of mist-netting over 3 days in areas where thornbills have been located.

Ensure that a suitably experienced and authorised person examines captured birds, because positive identification may require the bird to be in the hand.

Release birds promptly at the point of capture unless an authorised wildlife professional directs otherwise.

Recommended effort: 6 hours over 3 days, only where thornbills have been located by searches or transects

Survey methodology: 3. Timing and detection

The survey guideline gives no confirmed breeding season or detailed habitat information for the King Island subspecies, so plan surveys when suitable habitat can be safely searched and when access conditions allow the full effort to be completed.

Treat call records and visual observations as detection records, not as confirmation of the subspecies, unless the observer can reliably distinguish the longer bill or the bird is examined in the hand.

Record weather, start and finish times, search route, habitat searched, calls heard, birds seen, capture effort and the identity of the observer.

Do not substitute general Brown Thornbill records from mainland Australia or Tasmania for a King Island subspecies identification.

Key identification feature: The King Island subspecies has a substantially longer bill than Brown Thornbills from mainland Australia and mainland Tasmania

Known body size: Approximately 9 to 11.5 centimetres long and 7 grams

Survey methodology: Minimum effort and reporting

  • Complete 10 hours over 5 days of area searches or line transects at sites smaller than 50 hectares.
  • Complete 6 hours over 3 days of mist-netting where searches have located thornbills.
  • Map all suitable habitat, search effort, detections, capture locations and retained habitat on project plans.
  • Report records, including non-detections and any uncertain records, to the relevant Tasmanian wildlife database and the Australian Atlas of Living Australia where appropriate.
  • Retain photographs, call recordings, field notes, permit details and fauna handling records with the project environmental records.
  • Escalate any credible record to the relevant state threatened species authority and the Australian Government environmental regulator before continuing works in the detection area.

Before habitat disturbance: Avoid and minimise

Plan the work around the subspecies' extremely small and fragmented known range on King Island. Avoid removing dense native understorey, wet forest and tea-tree thickets wherever practicable.

  • Retain existing woodland, wet forest, dense mixed scrub and tea-tree thickets that could provide cover or foraging habitat.
  • Retain swamp vegetation and do not drain wetlands or thickets that support suitable habitat.
  • Keep connected patches of native vegetation and establish or retain habitat corridors between retained patches where clearing cannot be avoided.
  • Limit clearing, burning, grazing, trampling and other activities that reduce dense understorey.
  • Keep construction compounds, access tracks, spoil and stock away from retained habitat and its understorey.

Before habitat disturbance: Approvals and work planning

  • Check whether the proposed action may affect the EPBC Act listed King Island Brown Thornbill and obtain project-specific regulatory advice before clearing.
  • Confirm Tasmanian threatened species, wildlife handling and clearing requirements before works start.
  • Prepare a fauna management plan that identifies suitable habitat, survey effort, exclusion areas, fauna handling arrangements, stop-work triggers and reporting responsibilities.
  • Stage clearing so that retained habitat and potential movement routes remain available while each work area is completed.
  • Use the survey results and current records to update the work footprint before machinery enters suitable habitat.

Before habitat disturbance: Timing and site preparation

  • Allow enough time to complete the recommended searches and any follow-up mist-netting before vegetation removal.
  • Treat the breeding period of mainland Tasmanian Brown Thornbills, August to January, as a period requiring additional planning because the King Island subspecies has no confirmed breeding season.
  • Mark retained scrub, wet forest, paperbark thickets, swamp vegetation and proposed corridors on the ground before clearing.
  • Install exclusion fencing where it can protect retained habitat from machinery, vehicles, stock and construction access.
  • Brief machinery operators and contractors on the species' appearance, habitat and the stop-work procedure.

During habitat disturbance: Daily controls and spotter catcher duties

The fauna spotter catcher must have clear authority to stop work in suitable habitat. Machinery should work only within marked limits and must remain under direct control when vegetation or habitat is being removed.

  • Inspect exclusion fencing, retained habitat, access tracks and no-go areas at the start of each workday.
  • Complete a daily pre-start briefing covering the day's clearing area, machinery sequence, spotter catcher location, contact method and stop-work triggers.
  • Keep the fauna spotter catcher close enough to observe the clearing face and communicate directly with operators.
  • Clear in a controlled direction towards retained habitat only where this does not drive fauna into active machinery or unsafe areas.
  • Stop machinery immediately when the spotter catcher gives the stop signal, when an animal is seen, or when a marked habitat boundary is damaged.

During habitat disturbance: Habitat feature handling

  • Inspect dense shrubs, paperbark thickets, low vegetation, wet forest understorey and swamp edges immediately before removal.
  • Retain vegetation that can provide dense cover, and do not push felled material into retained scrub, wetlands or proposed habitat corridors.
  • Manage logs, rocks, burrows, nests and other fauna features under the approved fauna management plan if they are encountered, even though the supplied species information does not identify them as specific King Island Brown Thornbill requirements.
  • Prevent machinery, stock and construction traffic from trampling retained thickets and wetland vegetation.
  • Do not use fire or broad vegetation removal as a substitute for staged, inspected clearing.

During habitat disturbance: If a Brown Thornbill is found

  • Stop work within the immediate area and keep people, vehicles and machinery away from the bird.
  • Allow the bird to move into retained vegetation if it can do so safely and without being chased.
  • Do not catch, handle, band, transport or relocate the bird unless the person is authorised under the relevant permit and fauna management plan.
  • Use the longer bill and, where necessary, examination in the hand by an experienced authorised person to confirm the King Island subspecies.
  • If the bird is injured, contain it only under the authorised fauna procedure and transfer it to an approved veterinarian or wildlife carer as soon as practicable.
  • Relocate a bird only to nearby retained habitat that is identified in the fauna management plan and only under the direction of the authorised fauna handler.
  • Do not recommence work until the spotter catcher and project environmental officer agree that the animal has left the risk area and the record has been completed.

During habitat disturbance: Stop-work triggers and records

  • Stop work for any confirmed or suspected King Island Brown Thornbill in the clearing zone, any injured fauna, damage to an exclusion area, or any clearing outside the approved footprint.
  • Record the date, time, exact location, habitat, activity, people involved, identification basis, photographs, handling, release or transfer outcome and the time work restarted.
  • Notify the project environmental officer and the relevant state wildlife authority of a confirmed record or an injured animal.
  • Review the clearing method and exclusion area after every detection before work continues.

After habitat disturbance: Post-clearance checks

Post-clearing checks should confirm that no fauna remains in the work area and that retained habitat has not been damaged. Keep all records available for regulatory and project review.

  • Inspect the cleared area, retained vegetation edge, access routes and exclusion fencing after each clearing stage.
  • Check dense understorey and wetland margins for injured or trapped fauna before the next construction activity begins.
  • Confirm that no machinery, spoil, drainage or stock access has entered retained scrub, wet forest, tea-tree thickets or habitat corridors.
  • Document any damage to retained vegetation and repair the control before the next work stage.

After habitat disturbance: Monitoring and reporting

  • Repeat targeted area searches or transects where the project affects retained habitat, corridors or a location where the subspecies was detected.
  • Use the same routes, effort and habitat descriptions for repeat monitoring so results can be compared.
  • Submit confirmed records and relevant survey results to the relevant state wildlife database and provide records required under any EPBC Act approval or state permit.
  • Report unexpected deaths, injuries, repeated detections or loss of retained habitat to the project environmental officer and relevant authorities.

After habitat disturbance: Maintain controls and offsets

  • Keep exclusion fencing, no-go markers, drainage controls and weed controls in place until construction risks to retained habitat have ended.
  • Maintain any approved offsets or other compensatory measures for unavoidable habitat loss in accordance with the project approval.
  • Use monitoring results to adjust construction access, weed control, grazing controls, fire planning and revegetation where habitat condition declines.

Rehabilitation actions: Restore habitat structure and connectivity

Rehabilitation should rebuild dense native cover and reconnect retained habitat on King Island. The main focus is habitat structure, wetland protection and reducing the processes identified as threats to the subspecies.

  • Revegetate cleared areas with locally appropriate native woodland, wet forest, tea-tree and dense shrub species that recreate the habitat types recorded for the subspecies.
  • Prioritise corridors between retained native vegetation patches rather than isolated planting blocks.
  • Restore dense understorey suitable for cover and foraging, including vegetation similar to paperbark, prickly shrubs, wattles, banksia, heath and bracken communities recorded at Pegarah State Forest.
  • Do not rely on open grassland planting where the project has removed dense scrub or wet forest understorey.
  • Protect revegetation from grazing, trampling and browsing until dense cover is established.

Rehabilitation actions: Protect wetlands and manage threats

  • Prevent drainage of swamps and maintain wet conditions that support paperbark and tea-tree thickets.
  • Control weeds and prevent construction materials, sediment and stock from degrading restored wetland and scrub habitat.
  • Prepare a fire management approach that reduces the risk of uncontrolled fire removing the understorey across retained and restored habitat.
  • Manage grazing and burning so that dense native cover is retained.
  • Assess nearby Tasmanian Thornbill activity if a King Island Brown Thornbill is located, and obtain specialist advice before applying any competitor control.

Rehabilitation actions: Monitor rehabilitation success

  • Monitor planted areas and retained corridors for survival, native understorey development, weed cover, grazing or trampling damage and fire impacts.
  • Use repeat area searches or transects in suitable restored habitat once vegetation has developed enough to provide dense cover.
  • Record Brown Thornbill observations separately from confirmed King Island Brown Thornbill records, and seek in-hand confirmation where the subspecies cannot be identified confidently in the field.
  • Set project success measures around retention or establishment of dense native cover, protection of wetland thickets, functioning vegetation corridors and absence of uncontrolled grazing, drainage and fire damage.
  • Report confirmed detections and rehabilitation outcomes to the relevant state wildlife database and the approval authority where required.

Sources