However, reconstructions of the eggshell fragments, done by Natalie Schroeder of the South Australian Museum, reveal an unexpected, and unusual, fact: the eggs from what are thought to be Genyornis appear to be slightly smaller than modern emu eggs. If these truly were dromornithid eggs, they would be highly unusual – adult Genyornis were a whopping six times as large as a modern emu. This startling body-to-egg size ratio was an indication that perhaps the eggs being described as belonging to Genyornis were from an entirely different bird.
Besides emus and dromornithids, there were no truly big birds living on mainland Australia. Moas, a member of the dinornithidae family, were living at the same time on New Zealand and persisted until very recently. These included the tallest birds to ever live, with the absolute tallest reaching almost 4m high. Even the smallest moas – still big as far as birds go – laid larger eggs than the purported Genyornis eggs.
Recently, however, another big bird has been added to the roster of prehistoric Australia: Progura gallinacea. This bird was not a mihirung, or a moa, or an emu – in fact, it was a giant megapode. Megapodes exist today in the form of some of Australia’s more common ground birds, including the malleefowl and the brush-turkey. Unlike these, P. gallinacea was as tall as a kangaroo, and weighed as much as 7kg – three times as large as modern brush-turkeys. This size puts them close to scale with modern emus, so the similar egg size makes sense for this species.
Megapodes are known for their unique nests. Unlike most birds, megapodes construct giant mounds of soft, warm earth or sand to incubate their eggs. This style of incubation – burying one’s eggs and leaving them be – is much more similar to how turtles and many lizards incubate, rather than other birds. While Progura lacked adaptations for constructing such mounds, it appears that it might not have needed to. The abundance of eggshells in Australian sand dunes indicate that this bird was using these dunes as an even more convenient method of incubation – rather than burying them, laying their eggs tucked away in the dunes.
Although the mystery of the Genyornis eggs appear to have been resolved for now, it is clear that the giant megapodes suffered the same fate as the mihirungs. Shortly after humans arrived in Australia, the giant birds disappeared from the landscape. It is likely that these birds, having evolved on an island, were naïve to the newly-established predators, making them, and their giant eggs, easy targets for humans on the hunt.
Trevor H. Worthy. 2016. A case of mistaken identity for Australia’s extinct big bird. The Conversation.