The Australian zebra finch (Taeniopygia castanotis) is the most common estrildid finch of Central Australia. It ranges over most of the continent, avoiding only the cool humid south and some areas of the tropical far north. The bird has been introduced to Puerto Rico and Portugal. Due to the ease of keeping and breeding the zebra finch in captivity, it has become Australia's most widely studied bird; by 2010, it was the most studied captive model passerine species worldwide, by a considerable margin.
Taxonomy and phylogeny The Australian zebra finch was described in 1837 by John Gould as Amadina castanotis, about two decades after the Sunda zebra finch (T. guttata) was described. For over a century and a half, the Australian and Sunda zebra finches were classified as a single species, Taeniopygia guttata. They were split by the IUCN Red List and BirdLife International in 2016. The International Ornithological Congress followed suit in 2022 based on studies noting differences in plumage, mtDNA divergence, and assortative mating between both species in captivity.
It is likely that the Australian zebra finch evolved first, with the Sunda zebra finch descending from Australian zebra finches blown out to sea during the Pleistocene.
Habitat and distribution
The Australian zebra finch has the most extensive mainland distribution of the Australian estrilids, being found in about 75% of mainland Australia. The species is generally not found on the coasts, except for the arid western edge.
The Australian zebra finch is generally found in more arid areas. The areas it chooses to occupy are close to water, and places where rain is concentrated after it falls. However, this is likely more related to the abundance of vegetation than the abundance of water as a resource in itself. Within these areas, it is found in grasslands with scattered trees and shrubs, and in open or grassy woodlands. It is also found in cultivated areas, such as rice fields. It usually stays confined to the low coastal areas of the islands it inhabits, but it can move to elevations up to to exploit expanding cultivation and grasslands.
Although Australian zebra finch breeding, for example, is initiated by rainfall, Klaus Immelmann proposed that sustained heavy precipitation is detrimental to the zebra finch. This is supported by the observation that the nest does not shield the chicks or eggs from rain, and rainfall can sometimes result in clutches being abandoned. Furthermore, it is supported by Immelmann's finding that zebra finches left Wyndham after the first heavy rains in November 1959, but returned to breed in April. It is hypothesised that birds in parts of northern Australia migrate inland during the wet season from October to May, and return to the coastal regions during the dryer months.
Life cycle The life expectancy of an Australian zebra finch is highly variable because of genetic and environmental factors. The zebra finch may reach up to five years in its natural environment. If they are kept caged, they normally live for 5 to 9 years but may live as long as 12 years, with an exceptional case of 14.5 years reported for a caged specimen. The greatest threats to captive zebra finch survival are predation by cats and loss of natural food.
Song and other vocalisations Australian zebra finches are loud and boisterous singers. Their calls can be a loud beep, meep, oi! or a-ha!. Their song is a few small beeps, leading up to a rhythmic song of varying complexity in males. Each male's song is different, although birds of the same bloodline will exhibit similarities, and all finches will overlay their own uniqueness onto a common rhythmic framework. Due to their extremely fine temporal-auditory discrimination, the zebra finch is able to recognise and respond to micro-auditory details nested within their calls which human ears cannot detect.
Sons generally learn the song of their fathers with little variation. There is a critical sensitive period during which juvenile males learn their songs by imitating a mature, male tutor. Subsong (early, poorly structured vocalisations) evolve into 'plastic song'. This plastic song is variable between renditions but begins to incorporate some recognisable elements of tutor songs. A study conducted by Nottebohm et al., has shown that birds were able to successfully imitate their tutor's song after relatively short exposure (40 playbacks of the motifs lasting 30 seconds total) over the duration of their sensitive learning period. These birds eventually form a "template" of what their correct song should sound like. They rely on auditory feedback for both song learning and practice as juveniles and song maintenance as adults. Adult birds maintain their songs by correcting any deviations from their target song template. During adulthood, by around 90 days, the bird's song goes through a crystallisation phase where their song template is stable and it no longer changes. The learning process can be delayed by exposure to traffic noise.
Male Australian zebra finches begin to sing at puberty, while females lack a singing ability. This is due to a developmental difference, where in the embryo, the male zebra finch produces testosterone, which is transformed into estradiol in the brain, which in turn leads to the development of the nervous system for a song system. There are multiple areas of the brain involved in the production of song. When a bird is singing a learned song, the HVC projects to the robust nucleus of the archistriatum (RA), which itself projects to the hypoglossal motoneurons. These motoneurons control the muscles of the trachea and syrinx. When learning a new song, the HVC sends efferents to Area X in the lobus parolfactorius, which connects to the medial nucleus of the dorsolateral thalamus (DLM). This structure connects to the lateral magnocellular nucleus of the anterior neostriatum (LMAN), which projects to the RA, and continues like a normal learned song would. The function of the various areas involved in learning is still being investigated. Area X is likely involved in the acquisition of a new song, whereas the LMAN likely serves a key role in the plasticity necessary for learning. Activation of song behaviour later depends on androgens.
Because Australian zebra finch males learn their songs from their surroundings, they are often used as avian model organisms to investigate the neural bases of learning, memory, and sensorimotor integration. For example, studies have investigated the role of FoxP2 in song learning and have found that in young finches both knockdown and overexpression of FoxP2 in the striatal song control nucleus, Area X, prevents accurate song learning and tutor imitation. These studies also have implications for human speech. Individuals heterozygous for a point mutation in FOXP2 manifest a speech disorder. Because of similar expression patterns between humans and songbirds, the Australian zebra finch is used as a model to study FoxP2 expression and function. The zebra finch genome was the second bird genome to be sequenced, in 2008, after that of the chicken.Taeniopygia guttata Research Status. Washington University in St. Louis
The Australian zebra finch uses an acoustic signal to communicate to embryos. It gives an incubation call to its eggs when the weather is hot—above —and when the end of their incubation period is near. This call alters the growth and behaviour of the chicks, with chicks that were given an incubation call having less mass at the end of the nestling phase when they experienced higher nest temperatures. This contrasts with chicks that were not given an incubation call, which have a higher mass at the end of nestling after being exposed to high nest temperatures. Additionally, the chicks called to as an embryo are more likely to call after experiencing high nest temperatures.
Calling behaviour is used by Australian zebra finches to negotiate parental care duties. In an experiment that delayed the return of the males to the nest, it was found that the resulting duets were shorter and calls were made more often. This is the first species that vocal negotiation over parental care has ever been reported.
Behaviour
Breeding
The Australian zebra finch generally breeds in loose colonies of up to 50 nests (although the number of individuals in a colony can be up to about 230 when breeding, and around 350 when not), but it may nest solitarily. It usually places its nest in a thorny shrub poor tree, although it will nest in other structures, natural and artificial. Often, there are multiple nests in one shrub.
In non-arid parts of Australia, colonies are often occupied year-round. During cold days in the non-breeding season, members of these colonies usually feed in flocks for about two hours, breaking up into small groups to rest, preen, sing, and court before foraging again in a large flock until about one hour before sunset, when they return to the colony. On warmer days, some of the small groups return to the colony to build nests and perform their normal activities. During the breeding season, finches finding or building a nest often return after about an hour of feeding, and on cold days they are joined by those incubating or brooding young. Pairs that have not yet laid their eggs sometimes elect to court and mate in special "courting trees" before joining the flock. In the afternoon, most pairs engage in social activities, which often take place at "social trees". The zebra finch frequently does not breed where it was born; of the ringed birds that bred in the Danaher breeding colony () from 1985 to 1989, 24% of them were hatched from the colony or in the immediate vicinity. This natal dispersal is not sex-biased, unlike in most passerines. However, males between 36 and 50 days of age are more likely to disperse than females, although after this age, more females disperse than males. Predation is likely a major factor in coloniality; nests in the main colony suffer less predation than nests further away. Pairs with preyed-upon nests are significantly more likely to nest in a bush more than away from their previous nesting plant. Another large factor is where others nest: individuals are more likely to nest closer to conspecifics. In addition, reproductive success of conspecifics may play a role in where individuals nest; a study published in 2012 found that this finch was more likely to breed near nests with chicks older than six days (used as a proxy for reproductive success because they fledge about 87% of the time).
The Australian zebra finch builds both a roosting and breeding nest. The former is dome-shaped, has a large entrance on the side, and lacks an entrance tunnel. This nest helps the zebra finch conserve body heat (likely through its roof and walls and by allowing birds to huddle together): an individual in a roosting nest saves about 18% of the energy of one outside. The breeding nest (which generally ranges from about in length) has a small entrance followed by a tunnel about in diameter and up to in length, which conceals the contents of the nest, leading to the egg chamber, which has (from the outside) a diameter of ; the latter two are separated by a raised lip, preventing eggs from rolling out. This chamber often sits on an old nest; otherwise, a foundation consisting of many short, stiff stems over horizontal branches is constructed. The walls of the nest range in thickness from , with an outer layer of longer stiff and rough grass stems and an inner layer of shorter soft and fine stems. The egg chamber is also lined with soft material, such as wool and feathers. Both sites are defended during the day; but while a desperate bird is sometimes let in to the roosting nest during the night, the breeding nest is always guarded.