Giant Moa Bird Not as Big of a Beast as Previously Believed

An extinct, flightless behemoth known as the giant moa bird may actually not have been so giant after all, new research suggests.

The scientific name of the giant moa bird, Dinornis robustus, literally means robust strange bird, and the species was the largest of at least nine moa bird species that roamed New Zealand's rough terrain until going extinct about 500 years ago, likely due to overhunting.

A new analysis of the D. robustus bones led by University of Manchester scientists suggests the giant birds were not as robust as previously believed, and that they were likely less hefty creatures than the size of their bones suggests. The leg bones of the giant moa bird are similar to that of modern, fast-running flightless birds, whereas a much smaller species of moa bird from a different taxonomic family was found to have an extremely robust skeleton given its size.

"Our research suggests that this group of birds came up with several different solutions to deal with the problem of supporting the large body necessary to process a diet of coarse vegetation," said biomechanics researcher Charlotte Brassey, who led the study recently published in the journal PLOS One.

"We know that these species of moa were living together in the same locations, at the same time. So we don't think the differences we're seeing in leg robustness are adaptations to a particular habitat type," she said. "Instead it seems they were perhaps engaging in different behaviors, although both could deal with extremely rough terrain."

One challenge presented to the research team was accurately determining how much these moa weighed in life. For similar studies on other species, scientists can usually estimate body weight with accuracy based on the size, shape and weight of bones. But with the moa birds, their excessive leg size was not necessarily a good base to begin estimates of body weight.

"If we'd wanted to estimate the weight of a saber-toothed cat, no-one would have suggested measuring canine tooth length and then scaling up the tooth size of your standard tabby," Brassey said.

"That's because we know that the saber-toothed cat had unusually oversized canines compared to house cats. It wouldn't be a fair comparison, and you'd end up with a ludicrously high estimate of the body weight of the saber-toothed cat.

"The same was true for moa. We already knew that moa had disproportionately wide leg bones, yet previous estimates of their body mass had been based on those same bones which probably resulted in overestimates," she said.

To more accurately assess the moa's body mass, the researchers preformed full skeleton CT scans of the giant moa. as well as a smaller moa species to use as a point of comparison.

As the research team suspected, the new estimates of the giant moa's body mass were much lower than previous estimates. However, the bird was still extremely heavy at 200 kilograms (441 pounds).