Sylviornis was a huge flightless bird, standing up to 1.2–1.6 m (3.9–5.2 ft) tall, and weighing around 40 kg (88 lb) on average.^[3] In the 2016 study, its height in resting stance was estimated up to 0.8 m (2.6 ft), while its mass estimate decreased to 27–34 kg (60–75 lb).^[2] It is the most massive pangalliform known to have ever existed. It had a large skull with a high and laterally compressed beak surmounted by a bony knob. Its legs were rather short, but had strong toes with long nails. The skeleton has a number of peculiarities and differences that make Sylviornis stand apart from all other known birds: the clavicles were not fused to a furcula, the number of caudal vertebrae was very high, and the ribcage and pelvis were almost dinosaurian in appearance. The wings were reduced to small stubs.^{[citation needed]}

Native accounts believed to be based on Sylviornis describe a bird reddish in color, with a star-shaped calque on its head, and fast despite being flightless because it used its reduced wings for balance while running.^[4]

The anatomy of its skull suggests that it had a reduced optic lobes, with a well developed sense of smell and somatosensorial system, adapted for being active during twilight conditions (crepuscular) in search of food.^[5] The diet is unknown. Because of its beak morphology and chicken-like feet, some authors guessed that the species was a herbivore that fed on low vegetation and dug up roots and tubers, but others that it was a specialized invertebrate predator.^[4]

A large proportion—up to 50% in some deposits—of the remains found were from juvenile animals. Thus, it has been theorized that Sylviornis had a clutch of at least two, more probably closer to 10 eggs, and that the average lifespan was not much more than 5–7 years, which would be extremely low for such a large bird. It was thought that the bird did not incubate its eggs but built a mound similar to the megapodes. Tumuli on the Île des Pins which were initially believed to be graves were found to contain no human remains or grave goods, and it has been hypothesized that they were the incubation mounds of Sylviornis. As these mounds are up to 5 m (16 ft) high and 50 m (160 ft) wide even after nearly four millennia, they seem too large to have been made by the giant scrubfowl (Megapodius molistructor), an extinct New Caledonian species of megapode. However, recent assessment of this bird as outside and not even particularly closely related to megapodes make the possibility that it was a mound-builder like them strictly unlikely.^[2]

In native accounts, the bird only laid one egg between November and April that was not incubated, covered, or protected in any way. However, the adults were aggressive.^[4]

Sylviornis is the most common fossil animal in New Caledonia and its remains are often found in human contexts. The bird was likely hunted to extinction by the Lapita ancestors of the Kanak people, who settled New Caledonia around 1500 BCE. The most recent evidence of the species is a bone from the Pindai Caves carbon dated to 1120-840 BCE.^[6] If native accounts are accurate, its eggs and hatchlings would also be vulnerable to introduced mammalian predators.^[4]

• Biodiversity of New Caledonia
• Holocene extinction
• Island gigantism
• Late Quaternary prehistoric birds

• Mourer-Chauviré, Cécile; Balouet, J. C. (2005). "Description of the skull of the genus Sylviornis Poplin, 1980 (Aves, Galliformes, Sylviornithidae new family), a giant extinct bird from the Holocene of New Caledonia". In Alcover, J. A.; Bover, P. (eds.). Proceedings of the International Symposium "Insular Vertebrate Evolution: the Palaeontological Approach". Monografies de la Societat d'Història Natural de les Balears. Vol. 12. pp. 205–218.
• Poplin, François (1980). "Sylviornis neocaledoniae n. g., n. sp. (Aves), ratite éteint de la Nouvelle-Calédonie". Comptes Rendus de l'Académie des Sciences, Série D (in French). 290: 691–694.
• Balouet, Jean-Christophe; Olson, Storrs L. (1989). "Fossil Birds from Late Quaternary Deposits in New Caledonia" (PDF). Smithsonian Contributions to Zoology. 469 (469): 1–38. doi:10.5479/si.00810282.469.
• Worthy, Trevor H.; Mitri, Miyess; Handley, Warren D.; Lee, Michael S. Y.; Anderson, Atholl; Sand, Christophe (30 March 2016). "Osteology supports a stem-galliform affinity for the giant extinct flightless birds Sylviornis neocaledoniae (Sylviornithidae, Galloanseres)". PLOS One. 11 (3): e0150871. Bibcode:2016PLoSO..1150871W. doi:10.1371/journal.pone.0150871. PMC 4814122. PMID 27027304.

• Article with reconstruction pictures of Sylviornis