
Dromornis stirtoni

Dromornis stirtoni, colloquially known as Stirton's thunderbird, was a gigantic flightless bird of the Dromornithidae family. The remains of this "Dromornis" were discovered at the Alcoota Fossil Beds in the Northern Territory of Australia. It was first described in 1979 and is currently believed to be the largest avian species to have ever existed.
Taxonomy.
The description of this species, along with several other new dromornithids was published in 1979 by Patricia Vickers-Rich. This included a major revision of the family, described a century earlier in Richard Owen's 1874 publication of a new species and genus. Large amounts of fragmentary material is found at the Alcoota fossil site in Central Australia, the type location, is the only certain occurrence of the bird. Rich proposed the specific epithet for fellow palaeontologist Ruben A. Stirton, an American who undertook extensive research on Australian taxa.
Informal names for the species include Stirton's thunderbird and Stirton's mihirung.
Description.
It was three metres (10 feet) tall and weighed half a tonne (1100 lbs). It inhabited subtropical open woodlands in Australia during the Late Miocene and may have been carnivorous. Due to the poor fossil record of "Dromornis australis" (the type species of the genus) and the large time gap between the two "Dromornis" species, "D. stirtoni" may eventually be reassigned to the genus "Bullockornis".
This species had a long neck and stub-like wings, rendering it flightless. Its legs were powerful, but it is not believed to have been a fast runner. The bird's beak was large and immensely powerful, leading early researchers to believe that it was used to shear through tough plant stalks. However, recently others have argued that the size of the beak suggests that the bird was a carnivore.
The weight and height of the animal is thought to have been exceedingly large. Peter F. Murray and Patricia Vickers-Rich, in a chapter of their work "Magnificanet Mihirungs" (2004), studied evidence used three different methods to conclude that the mass was probably up to 500 kilograms, making this the heaviest bird known by exceeding the ratite elephant birds of the Aepyornithidae family. "Dromornis stirtoni" was compared to "Aepyornis maximus" by the authors, the largest of that family. The height of "D. stirtoni" would probably have exceeded the tallest species of the genus "Dinornis", the giant moas of New Zealand.
Comparison of two partial crania with the near complete cranium of "Dromornis planei" ("Bullockornis") shows the head of this species to be about 25% larger. Reconstruction of overlapping remains of the rostrum have revealed its form and size, the lower mandible would have been around 0.5 metres. The size and proportions of the head and its bill are comparable to that of mammals such as camels or horses.
Two forms of unearthed specimens are considered to be due to strong sexual dimorphism, concluded in a 2016 morphometric analysis using landmark based and actual measurements which also supported earlier conclusions regarding the species enormous size. This histological technique has been applied to other large and extinct avian species, including investigation into the paleobiology of the elephant birds Aepyornithidae. Males were comparatively more robust and heavier, a mean estimated at 528 kg, although not necessarily taller than females, at 451 kg.
The disparity in robustness was interpreted by the researchers as evidence of the biology of the species, behaviours such as incubation by the female, pair bonding, parental care and aggression while nesting, and courtship or display habits exhibited by extant waterfowl, the anseriforms.
Habitat.
Evidence of its geographic distribution is restricted to Alcoota Station in the central region of Australia's Northern Territory.
The temporal distribution is the late Miocene.
The vegetation type of the region in that period was open woodland favouring its subtropical climate.
"Dromornis stirtoni" is found amongst the depositions of the Alcoota and Ongeva Local Faunas, dated to the Late Miocene and early Pliocene.
Fragmentary remains are common at these sites, although little is assignable to an individual of the species. Some depositions contain fragments of around four individuals in disarray over an area of one square metre.
Other dromornithid species have been found alongside this species, "Ilbandornis woodburnei" and the tentatively placed "Ilbandornis lawsoni", resembling the large but more gracile modern birds such as ostriches and emus.
"Dromornis stirtoni" probably existed in an assemblage of fauna that included other dromornithids and browsing marsupials as the apex herbivore. The Alcoota Local Fauna were deposited at the only known upper Miocene fossil beds of Central Australia. The early conceptions of a fearsome bird receives some support from the proposed behaviour of the larger males aggressively defending a preferred range against competitors, other males or herbivores, and predators.
Feeding and diet.
It is widely accepted that "Dromornis stirtoni" was a herbivorous bird, predominantly eating seed pods and tough-skinned fruits. This has been deduced from various key features of the bird. One of these features is that the end of the bird's bill, does not have a hook, and that the beak is instead wide, narrow and blunt, typical of a herbivore (Monroe, 2009). The bird also had hoof-like feet, rather than 'talons', which are typically associated with carnivores or omnivores. Moreover, placement of the eyes on the side of the birds head prohibits it from seeing directly in front, which would limit its ability to hunt. Lastly, analysis of the amino acids within the egg shells of "Dromornis stirtoni" suggest that the species was herbivorous. Despite this however, there are various indicators that suggest the bird may have been carnivorous or omnivorous (Murray, 2004). The size and muscularity of the birds skull and beak would also suggest that they may not have been herbivores, as no source of vegetable food in their environment would have required such a powerful beak (Vickers-Rich, 1979). In recognition of the varying opinions, it is widely accepted that whilst the large bird may have occasionally scavenged or eaten smaller prey, they were mostly herbivorous.
Extinction.
It is proposed that various factors may have contributed to the extinction of "Dromornis stirtoni". Palaeontologists Murray and Vickers-Rich suggested that the diet of "Dromornis stirtoni" may have overlapped considerably with the diets of other large birds and animals, and that the subsequent converging trophic morphology could have contributed to the large birds extinction as it was 'out-competed' of its food source. 
Alternate arguments have proposed that the large birds' breeding patterns may have contributed. This suggested that "Dromornis stirtoni" lived for a relatively long period of time in a group of older birds, however, for the few young that were produced, time to maturity was considerable. Subsequently, breeding adults were replaced slowly, which left the species highly vulnerable if breeding adults were lost.
It is thought that these factors may have worked in conjunction with the introduction of humans, and the predation and environmental modification that was associated. Whilst direct evidence of human predation of "Dromornis stirtoni" is limited, various experts propose that the large bird may well have been hunted, recognising that more than 85% of Australian mega fauna became extinct during the same period that humans were introduced. The use of 'fire-stick burning' as an ecosystem management technique is thought to have adversely impacted the semi-arid climates within which Dromornis Stirtoni lived, as the ecosystem lacked the required resilience, and as such, food supplies were impacted.