Our national bird as you've never seen it

Incredible new footage has revealed a rare glimpse into the unseen behaviours of our quirky national bird - and in this case, it's a bit of biffo.

The two male kiwi were captured having a violent stoush amid the undergrowth in an undisclosed offshore sanctuary.

After climbing out from a burrow, tensions between the pair quickly boil over, giving way to a noisy chase around the forest.

Dr Isabel Castro, who helps oversee the Behavioural Ecology and Conservation Programme run by Massey University, said the reason for the scrap was simple - sex.

"We were very lucky to see this. As with a lot of behaviour, you don't often see it when you are there because these birds don't like us being there and will try to hide."

Filmed by six cameras during the last breeding season, the video revealed how interactions between males could become hostile when they were competing for mating partners.

"It means testosterone levels are very high, and we are seeing the birds aggressively fighting with each other while trying to choose who they are going to mate with," Dr Castro said.

"The male kiwi know that time is the chance - and if they don't fight then, they won't get the girls."

The programme monitors around 50 kiwi, which are tracked by transmitters attached to them, as well as cats.

Dr Castro said the clip would feature as part of a new documentary series by famed naturalist Sir David Attenborough, titled Attenborough's Big Birds.