Scientists unravel mystery behind extinction of world’s largest bird

New Zealand’s moa, a group of giant flightless birds including the largest birds ever to have lived, died out because they grew almost ten times slower than living species, reveals a study by researchers at the Zoological Society of London’s Institute of Zoology (IoZ) in London.

The new study published today in the journal Nature, in collaboration with the University of Oxford and the University of Canterbury in New Zealand, analysed moa leg bones and discovered up to nine growth rings (similar to tree rings) in bone cross sections, revealing that moa took almost a decade to reach their adult body weight. Moa then took several more years to reach sexual maturity. This is very different to all living birds, which all reach adult size within 12 months.

“This study gives a fascinating insight into the growth rate of birds in the absence of mammalian predators,” says Dr Sam Turvey of the IoZ. “New Zealand’s unique environment prompted the evolution of ‘delayed maturity’ in moa, enabling them to grow slowly over a long period of time.”

New Zealand, the world’s most isolated major landmass, was once home to predatory giant eagles but lacked any mammalian predators prior to the arrival of humans around 1300 AD. It is thought that the long growth period of moa evolved in response to this almost predator-free environment. Moa also became giant-sized in the absence of mammals – the largest moa, Dinornis, measured over two metres tall and weighed a quarter of a ton.

Dr Turvey continued “Moa are closely related to many living birds, including ostrich, emu and cassowary, which suggests that all birds may have this inherent ability for extending their development in this way. However, if birds evolve in the presence of predators, the emphasis is instead on more rapid reproduction.”

Moa almost certainly died out within 100 years of Maori settlers colonising New Zealand – it seems that they simply couldn’t grow fast enough to breed and replenish their heavily hunted population.