Australian zebra finch

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 2,300 metres (7,500 ft) 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.

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 26 °C (79 °F)—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.

Breeding