Black-tailed Nativehen Tribonyx ventralis Scientific name definitions

30–38 cm; male 250–530 (412) g, female 322–405 (364) g; wingspan 55–66 cm. Large , thickset rail with vertically fanned black tail , black undertail-coverts, long wings, orange-yellow iris , nimble gait and somewhat bantam-like shape; gregarious; swims readily, with buoyant carriage and erect tail; these characters, and preference for foraging in groups in drier habitats, distinguish from all sympatric rallids. <em>T. mortierii</em> is much larger, flightless and appears much shorter-winged; has red iris, olive-yellow bill and grey or grey-olive legs and feet. Sexes similar, but female slightly smaller, and slightly duller and paler overall; dark band round base of bill reduced or absent and white spots on sides of belly smaller. Immature very similar to adult, but some retain smaller juvenile white flank spots; bare part colours identical to adult. Juvenile resembles adult female but has smaller white flank spots; in worn plumage is paler, with off-white lores, face, chin and throat, and paler underparts contrasting more with blackish vent and undertail-coverts; bill greenish yellow with dusky tip; iris blackish; legs and feet brownish pink.

Australia (mostly inland).

Opportunistic, especially during influxes. Normally occurs in low rainfall areas at permanent or ephemeral terrestrial wetlands, including shallow lakes, swamps, pools, floodplains and flats of rivers and creeks, and inundated depressions; favours fresh or brackish waters but often on shallow and more saline drying ephemeral wetlands; rarely in dry Banksia woodland, and at mangroves and tidal pools. Habitat often characterized by dense clumps of lignum, canegrass, or scrub; sometimes sparsely wooded. Also occurs at artificial wetlands, e.g. dams, margins of reservoirs, sewage ponds; in pasture, crops or fallow lands. May occur in urban areas, e.g. streets, gardens, parks, golf courses and racecourses; and in unusual situations such as arid country, sandhills, undulating hills and valleys, coastal flats, and among samphire and coastal shrubs. May occur far from water.

May make regular seasonal movements: reported as regular visitor in extreme SW, and reporting rates in N increase in summer; however in some areas present at most times of year. Dispersive and highly irruptive; since 1833 irruptions have occurred on average once every 2·7 years. Irruptions probably associated with favourable breeding conditions in N allowing build-up of numbers, followed by harsh conditions such as drought and decreasing food supplies which force birds to leave interior. Irruptions sometimes characterized by sudden appearance and disappearance of large numbers of birds over period of as little as 12 hours. Movements often linked with rain: often appears after rain and after cyclonic disturbances; also claimed to appear before rain actually falls; possibly moves in response to cloud banks. May move into flooded areas and areas with lush green growth, and away from areas subjected to drying-out, drought or hot weather. During irruptions possibly disperses in all directions, but in Queensland may move towards coast. Possibly moves back to normal range after rainfall. May move at night, and can fly long distances. Vagrant to Tasmania (3 records) and New Zealand  ; these and mainland vagrant occurrences often coincide with irruptions.

Takes seeds, plant material and invertebrates (especially insects). Plant food includes grain, seeds of Polygonum, Triticum and Hordeum, young vegetables and cereal crops, fodder crops, fresh green growth of annuals, aquatic plants, and drying apricots; invertebrate food includes molluscs, and adult and larval insects (Orthoptera, Coleoptera, Lepidoptera, and Hymenoptera in form of ants). Diurnal ; forages gregariously or in pairs ; feeds at grassy or muddy wetland margins , on open ground near wetlands, in pastures and crops, and in scrub and sand dunes; sometimes feeds with domestic fowl and at piggeries. Gleans from ground , alternately running and stopping in order to disturb insects. Also feeds from surface of water and submerges head and shoulders.

E & S Australia, usually Aug–Dec, but timing influenced by rainfall, especially after drought, when may breed any month; in SW, Jun–Nov; in SE breeds after large influxes, continuously through winter and summer; breeds opportunistically, often soon after arrival following heavy rain, or as water dries up after flooding; laying time correlated with peak rainfall + 2–3 months. Social organization unclear; on existing evidence, appears monogamous, and territorial when breeding; often nests in colonies. Nest usually well concealed low in dense vegetation, such as clumps of grass, reeds, bushes, and herbage; also on tree stump, in tree fork or branches, on debris on log in water, in shallow depression in ground, in wooden box in tree, and under wire fence enclosure. Nest cup-shaped, of any available vegetation, including lignum, reeds, twigs, leaves, grass and bark; often with partial roof of woven stems; sometimes with approach ramp of broken reeds; external diameter 26 cm, depth 10 cm; height above ground 0–80 cm. Usually 5–7 eggs (4–12); larger clutches possibly from 2 females; laying probably synchronous in large colonies where arrival virtually simultaneous; incubation probably begins before clutch complete, period unknown; hatching asynchronous but probably within 24–48 hours; chicks precocial and nidifugous; black downy chick has greenish sheen, colour fading to black-brown on body, bill black with large white egg-tooth, pink basal third of upper mandible and narrow white or grey central saddle, iris blackish, legs and feet grey-black; chicks remain in nest for unknown time after hatching; timing of chick development not recorded, and no information on parental care; date of post-juvenile moult presumably depends on hatching date, recorded Jan–Apr, but birds with fresh plumage and no moult collected May and Jul.

Not globally threatened (Least Concern). Generally widespread inland S of 20° S and W of Great Dividing Range, with scattered records from coastal and sub-coastal regions. Australian Atlas records no breeding N of 22° S. Locally common; wide fluctuations in numbers recorded, large concentrations of 10,000–20,000 birds recorded during irruptions. Annual indices of relative abundance, obtained in 1983–1989 and covering wetlands in c. 12% of land area in E Australia , were between 2222 and 25,424 (61–100% of total numbers were counted). Irruptions more frequent 50–100 years ago; lower frequency in recent years possibly because of reduced extent of breeding grounds following drainage of swamps and control of flooding on inland rivers. During irruptions, causes damage to crops and vegetable gardens by trampling and eating plants, and may pollute water supplies. Artificial habitats are often used, especially during irruptions.