Kiwis have turned out to be smarter than expected - especially compared to their extinct cousins, the moas.
Dr Fabiana Kubke, of the University of Auckland's anatomy department, said research using MRI imaging and CT scanning showed kiwis had surprisingly big brains for their body size.
He added that kiwi brains were similar to "smarter" flighted birds whereas the long-lost moa had a significantly smaller brain, like other flightless birds such as ostrich and emu.
Dr Kubke said the brain of a 2kg adult kiwi averaged about 10 grams in weight.
By comparison the brain of an emu weighed about 25 grams, but the bird weighed about 40kg.
It was estimated the brain of an extinct giant North Island moa weighed about 42 grams for close to a 100kg body weight.
The relatively large size of the kiwi brain could hint at a higher intelligence as intelligent animals tended to have larger brains.
Intelligence also depended on how relative parts of the brain worked together, she said.
"In humans 85 per cent of the brain is the forebrain and we have a similar situation with the kiwi of 82 per cent," Dr Kubke said.
She said the kiwi forebrain was about twice as big as what might be expected.
It was possible the parts of the kiwi brain used for smell and touch could be larger than in other birds due to its nocturnal lifestyle.
"It is a different challenge to navigate in a dark environment."
Researchers had examined the brain size of several species of "old birds" (Palaeognaths), including kiwi, ostrich, emu, South American tinamou and the extinct New Zealand moa.
The results, published in the journal Brain, Behaviour and Evolution, showed the brains of different species of Palaeognath birds were smaller except in the case of the kiwi which had a similar brain-body ratio to "new birds" (Neognaths).
"In fact, the size of the kiwi cerebral hemisphere compared to total brain size is rivalled only by a handful of smart birds, such as songbirds and parrots, despite a lack of any known advanced behavioural abilities in kiwi."
Kiwi: 10g (for a 2kg bird)
Emu: 25g (for a 40kg bird)
Moa: 42g (for a 100kg bird)