Australian Little Bittern
Encyclopedia
The Black-backed Bittern (Ixobrychus dubius), also known as the Black-backed Least Bittern or Australian Little Bittern, is a little-known species of heron
in the Ardeidae family found in Australia
and southern New Guinea
. Only recently separated from the Little Bittern as a full species, it is one of the smallest herons in the world.
(Ixobrychus minutus), or of the New Zealand Little Bittern
(Ixobrychus novaezelandiae). However, molecular evidence has shown that it is more closely related to the Yellow Bittern
(Ixobrychus sinensis) than to the African and Palaearctic forms of the Little Bittern and is now recognised as a full species.
are yellow, the bill is yellow with a black culmen, and the feet and legs greenish-yellow.
, as well as patchily along the east coast, and in south-west Western Australia where it is locally common on the Swan Coastal Plain
. There are some scattered records from elsewhere, including coastal locations in the Kimberley region, the Top End
and the Torres Strait
islands, with vagrants occasionally reaching Lord Howe Island
and New Zealand
.
Apart from records of vagrants, circumstantial evidence that at least part of the population makes long-distance seasonal migrations is that most records, and the highest reporting rates, of the bitterns in southern Australia occur in spring and summer, with the birds largely absent in autumn and winter. There are few breeding records from the Australian tropics.
from November to April, only being reliably recorded elsewhere on the island from the lowlands of the Trans-Fly region
. There are no firm breeding records from New Guinea, though a specimen taken in the middle Fly River
lagoons was of a bird ready to lay, indicating breeding in the Fly River marshes in September. The lack of good breeding records suggests that at least some birds in New Guinea are seasonal migrants from Australia.
s, where they inhabit dense emergent vegetation of reeds and sedges, and inundated shrub thickets. They are also occasionally found in brackish and saline wetlands such as mangrove
swamps, Juncus
-dominated salt marsh
and the wooded margins of coastal lagoons.
by both parents. Young birds may start clambering in the reeds from 9–10 days old, taking their first flight when 25–30 days old, and remaining dependent on their parents for at least another 14 days.
s such as crustacean
s and dragonfly
larva
e, sometimes small vertebrate
s such as fish
and frogs. They hunt by waiting for potential prey animals to come within range of their bills, or by active stalking.
nationally in Australia, and as Endangered in Victoria. The global population has been estimated to comprise about 5000 mature individuals, mainly within Australia, including a sub-population of more than 1000 individuals in south-western Western Australia. Threats include various ongoing wetland degradation factors such as salinisation, drainage and the diversion of water for irrigation, as well as the destruction of nesting habitat by inappropriate burning regimes.
Heron
The herons are long-legged freshwater and coastal birds in the family Ardeidae. There are 64 recognised species in this family. Some are called "egrets" or "bitterns" instead of "heron"....
in the Ardeidae family found in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and southern New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
. Only recently separated from the Little Bittern as a full species, it is one of the smallest herons in the world.
Taxonomy
The bittern has sometimes been regarded as a subspecies of the Little BitternLittle Bittern
The Little Bittern is a wading bird in the heron family Ardeidae, native to the Old World, breeding in Africa, central and southern Europe, western and southern Asia, and Madagascar. Birds from temperate regions in Europe and western Asia are migratory, wintering in Africa and further south in...
(Ixobrychus minutus), or of the New Zealand Little Bittern
New Zealand Little Bittern
The New Zealand Little Bittern is an extinct and enigmatic species of heron in the Ardeidae family. It was endemic to New Zealand and was last recorded alive in the 1890s....
(Ixobrychus novaezelandiae). However, molecular evidence has shown that it is more closely related to the Yellow Bittern
Yellow Bittern
The Yellow Bittern is a small bittern. It is of Old World origins, breeding in much of the Indian Subcontinent, east to Japan and Indonesia. It is mainly resident, but some northern birds migrate short distances...
(Ixobrychus sinensis) than to the African and Palaearctic forms of the Little Bittern and is now recognised as a full species.
Description
At about 30 cm in length (range 25–36 cm), and with a weight of about 84 g (range 60-120 g), it is a very small bittern. The adult male has largely black upperparts, including a black cap, while the underparts, as well as the neck, breast and the sides of the head, are rich chestnut. There are large buff patches on the shoulders, conspicuous in flight. The female is duller, brown and streaked on back and crown; immature birds are similar. The iridesIris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
are yellow, the bill is yellow with a black culmen, and the feet and legs greenish-yellow.
Australia
In Australia the bittern is found in the south-east of the continent, with most records deriving from the Murray-Darling BasinMurray-Darling Basin
The Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
, as well as patchily along the east coast, and in south-west Western Australia where it is locally common on the Swan Coastal Plain
Swan Coastal Plain
The Swan Coastal Plain in Western Australia is the geographic feature which contains the Swan River as it travels west to the Indian Ocean. The coastal plain continues well beyond the boundaries of the Swan River and its tributaries, as a geological and biological zone, one of Western Australia's...
. There are some scattered records from elsewhere, including coastal locations in the Kimberley region, the Top End
Top End
The Top End of northern Australia is the second northernmost point on the continent. It covers a rather vaguely-defined area of perhaps 400,000 square kilometres behind the northern coast from the Northern Territory capital of Darwin across to Arnhem Land with the Indian Ocean on the west, the...
and the Torres Strait
Torres Strait
The Torres Strait is a body of water which lies between Australia and the Melanesian island of New Guinea. It is approximately wide at its narrowest extent. To the south is Cape York Peninsula, the northernmost continental extremity of the Australian state of Queensland...
islands, with vagrants occasionally reaching Lord Howe Island
Lord Howe Island
Lord Howe Island is an irregularly crescent-shaped volcanic remnant in the Tasman Sea between Australia and New Zealand, directly east of mainland Port Macquarie, and about from Norfolk Island. The island is about 11 km long and between 2.8 km and 0.6 km wide with an area of...
and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
.
Apart from records of vagrants, circumstantial evidence that at least part of the population makes long-distance seasonal migrations is that most records, and the highest reporting rates, of the bitterns in southern Australia occur in spring and summer, with the birds largely absent in autumn and winter. There are few breeding records from the Australian tropics.
New Guinea
In New Guinea the bittern is found seasonally in the reedbeds of the Waigani Swamp near Port MoresbyPort Moresby
Port Moresby , or Pot Mosbi in Tok Pisin, is the capital and largest city of Papua New Guinea . It is located on the shores of the Gulf of Papua, on the southeastern coast of the island of New Guinea, which made it a prime objective for conquest by the Imperial Japanese forces during 1942–43...
from November to April, only being reliably recorded elsewhere on the island from the lowlands of the Trans-Fly region
Western Province (Papua New Guinea)
Western Province is a coastal province in southwestern Papua New Guinea, bordering the Indonesian province of Papua. The provincial capital is Daru, on the island of Daru. The largest town in the province is Tabubil...
. There are no firm breeding records from New Guinea, though a specimen taken in the middle Fly River
Fly River
The Fly at , is the second longest river, after the Sepik, in Papua New Guinea. The Fly is the largest river in Oceania, the largest in the world without a single dam in its catchment, and overall ranks as the twenty-fifth largest river in the world by volume of discharge...
lagoons was of a bird ready to lay, indicating breeding in the Fly River marshes in September. The lack of good breeding records suggests that at least some birds in New Guinea are seasonal migrants from Australia.
Habitat
The birds are mainly found in freshwater wetlandWetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....
s, where they inhabit dense emergent vegetation of reeds and sedges, and inundated shrub thickets. They are also occasionally found in brackish and saline wetlands such as mangrove
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...
swamps, Juncus
Juncus
Juncus is a genus in the plant family Juncaceae. It consists of some 200 to 300 or more species of grassy plants commonly called rushes...
-dominated salt marsh
Salt marsh
A salt marsh is an environment in the upper coastal intertidal zone between land and salt water or brackish water, it is dominated by dense stands of halophytic plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh...
and the wooded margins of coastal lagoons.
Behaviour
Black-backed Bitterns are solitary, secretive and seldom seen, mainly active at dusk or at night. They are skulkers of reedbeds, walking in a crouched posture with head extended forward, crossing patches of open ground rapidly, stalking their prey at the water’s edge. When alarmed they will assume the cryptic posture typical of many bitterns, standing still with the head and bill extended vertically upwards. Usually reluctant to fly, when flushed they will do so with retracted head and dangling legs, skimming low over the water and the wetland vegetation.Breeding
The bitterns breed in spring and early summer, nesting in single pairs, or occasionally in loose colonies with the nests 15–30 m apart, in dense wetland vegetation. The nest is a platform of reeds and other plant matter, about 15–20 cm across and 10 cm thick, supported by the vertical stems of growing reeds, always situated over water and where there is overhead cover. The clutch usually comprises 4-6 matt white eggs, with an incubation period of about 21 days. The chicks are covered with orange-buff down and are fed by regurgitationRegurgitation (digestion)
Regurgitation is the expulsion of material from the mouth, pharynx, or esophagus, usually characterized by the presence of undigested food or blood.Regurgitation is used by a number of species to feed their young...
by both parents. Young birds may start clambering in the reeds from 9–10 days old, taking their first flight when 25–30 days old, and remaining dependent on their parents for at least another 14 days.
Feeding
The birds feed mainly on aquatic invertebrateInvertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s such as crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s and dragonfly
Dragonfly
A dragonfly is a winged insect belonging to the order Odonata, the suborder Epiprocta or, in the strict sense, the infraorder Anisoptera . It is characterized by large multifaceted eyes, two pairs of strong transparent wings, and an elongated body...
larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e, sometimes small vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s such as fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and frogs. They hunt by waiting for potential prey animals to come within range of their bills, or by active stalking.
Voice
Vocalisations of the bittern are poorly known. Outside the breeding season the birds are usually silent, though they may utter a small range of low croaking or grunting sounds. During the breeding season the advertising call, a deep, low and monotonous croaking call, repeated at half-second intervals and said to be made only by the males, may be heard.Status and conservation
Black-backed Bitterns are listed as Near ThreatenedNear Threatened
Near Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...
nationally in Australia, and as Endangered in Victoria. The global population has been estimated to comprise about 5000 mature individuals, mainly within Australia, including a sub-population of more than 1000 individuals in south-western Western Australia. Threats include various ongoing wetland degradation factors such as salinisation, drainage and the diversion of water for irrigation, as well as the destruction of nesting habitat by inappropriate burning regimes.
External links
- Birds Australia: Bittern Survey (includes downloadable call of Australian Little Bittern)