"In general, the heavier an animal species is, the lower its population density, or the numbers of a species that can be supported in an area of land," Latham said.
"This is because larger animals need more energy resources to survive. This relationship is strong for living species of flightless birds and therefore it can be used to estimate densities of extinct species of flightless birds".
Latham and colleagues compiled a global data set of body masses and population densities for living flightless birds and examined the statistical relationship between these variables.
They took in the effects of limiting factors such as the suitability of the habitat where populations occur and whether densities had been artificially reduced, such as by introduced predators or human disturbance.
They then applied this approach to extinct species by estimating population densities and population sizes for each of the nine species of moa to build up a picture of total moa density across New Zealand about 1000 years ago.
"We calculated a mean national density of moa ranging between two and 10 birds per square km – to give a total estimated population of between 0.5 and 2.5 million birds," co-author Jamie Wood said.
"These estimates reconcile previous population density estimates for moa, which have been either quite low or high."
While these new estimates were broadly similar to contemporary densities of large herbivorous wild mammals, such as deer, introduced into New Zealand about 150 years ago, the authors pointed out that "similar densities do not mean similar impacts, as many other factors affect the way large herbivores affect their environments".
It was hoped that the refined model can also allow improved population density estimates for other large prehistoric flightless birds around the world, such as the elephant birds from Madagascar, the Mauritian dodo, or the Hawaiian moa‐nalos.
"Population sizes and densities are important aspects of the ecology of extinct species, but are often overlooked because they are difficult to estimate," Latham said.
"But such data are key, not only for understanding the roles these species once played within their ecosystems, but also in understanding the processes that ultimately led to their extinction."
Moa insights
• The recent sequencing of the moa's genetic make-up for the first time suggested the bird likely didn't become wingless due to gene loss, or pseudogenisation. Having the moa nuclear genome could help studies of regulatory changes linked with flightless creatures, including ratites such as our kiwi.
• In 2017, scientists suggested moa would make poor candidates for resurrection - something wildly proposed by Labour's Trevor Mallard - and that, if it was being considered, more recently extinct species like the huia might be better options.
• In 2014, it was revealed how the moa was more closely related to South America's tinamous than its old bushmate, the kiwi, and both moa and kiwi separately evolved to become flightless after their ancestors flew here.
