Scientists work out how fast moas, extinct giant flightless birds, grew up by looking at the growth rings in their fossilised leg bones. This is similar to how the age of a tree is worked out, by looking at the growth rings in tree trunks.
Reported on the National Geographic website, Samuel Turvey of the Zoological Society of London said ‘Many living animals, such as polar bears, have these markings today. The rings result from the different growth rates over the course of the seasons.’
‘In the spring and summer there's more food being eaten and faster bone growth. If [the weather] becomes harsher, then the growth slows down. The changes in growth are reflected by changes in bone density,' Turvey explained.
By examining the rings in cross-sections of fossil moa bones, scientists found that most species of moa grew very slowly, some taking more than ten years to reach adult size. By contrast, most modern birds, even ostriches, tend to reach full size within a year.
Moas would have been very vulnerable when the first humans arrived in New Zealand, thought to be around 700 years ago. They were hunted by the human settlers and, as they reached maturity so slowly, would not have been able to reproduce quickly enough to maintain their populations - leaving them vulnerable to extinction. All moas were extinct by the time Europeans arrived in New Zealand in the 1760s.
Moas were unique to New Zealand, and the largest species, able to stretch up to over three metres in height, was the tallest bird in the world. Moa were first identified in 1839 by the famous naturalist, Richard Owen, after he was sent a strange fragment of fossilised bone to examine. He claimed it had come from a completely unknown giant bird, and was later proved correct when huge bone specimens were found.
Dr Joanne H Cooper, bird expert at the Natural History Museum adds, 'This discovery is really exciting because it shows us that even after 150 years of research into moa, we still have a lot to learn about this unique group of birds'.
The research findings of Turvey and his colleagues are published in the journal Nature .