Sylviornis is an extinct genus of galliform bird containing a single species, S. neocaledoniae, or erroneously, "New Caledonian Giant Megapode". Technically, the latter is incorrect because it has recently been found not to be a megapode, but the sole known member of its own family, the Sylviornithidae; at the time of its description, it was believed to be a ratite. Sylviornis was never encountered alive by scientists, but it is known from many thousands of subfossil bones found in deposits, some of them from the Holocene, on New Caledonia and the adjacent Île des Pins.
Sylviornis was a huge, flightless bird, 1.70 meters long altogether, and weighing around 30 kg. 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.
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. Apparently, 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 in reality the incubation mounds of Sylviornis. As these mounds are up to 5 m high and 50 m wide even after nearly 4 millennia, they seem too large to have been made by the Giant Scrubfowl, an extinct New Caledonian species of megapode.
Little can be said about the lifestyle of Sylviornis. It was probably a slow-moving browser, and the structure of the bill and feet suggest that roots and tubers it dug up formed a major part of its diet.
The bird was hunted to extinction by the Lapita ancestors of the Kanak people, who settled New Caledonia around 1500 BC. Predation by feral dogs and pigs probably also played a part. The legacy of Sylviornis persists in Kanak oral history in the form of stories giving a rough description of the bird and some of its habits. A native name was du.