A post-mortem has shown Jimmy the kākāpō died from complications following an X-ray.

Only 151 kākāpō remain in New Zealand following the death of Jimmy – a beloved member of the critically endangered species.

Department of Conservation (DOC) kākāpō expert Dr Andrew Digby said Jimmy died on December 12 as a result of complications following a veterinary procedure.

DOC suspected Jimmy had a broken wing, so flew him from Whenua Hou island to get a check-up in Invercargill. An X-ray cleared him of broken bones and Jimmy returned home.

But a post-mortem has since found the intubation tube used for inserting anaesthetic exacerbated the situation, leading to Jimmy's death.

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"It's not completely uncommon, but it's one of the risks with anaesthetic," Digby said. "This is normally a very low risk procedure, so we decided it was worth it in this case."

TWITTER/ANDREW DIGBY Kākāpō expert Dr Andrew Digby says Jimmy's death is "particularly devastating" because he hadn't produced offspring.

Jimmy was one of the 63 original birds on Whenua Hou island – flown there from Stewart Island in 1983 to help grow the population.

Only 32 of the founder birds are left, and Digby said Jimmy was one of the older ones.

TWITTER/ANDREW DIGBY Kākāpō expert Dr Andrew Digby: "Jimmy was one of my favourite kakapo. We’ll learn from this."

"Unfortunately, he had never produced offspring. He had mated before but never produced chicks.

"It's a real shame he hasn't been able to pass on his genetic material."

It was a "double-whammy loss" for the kākāpō population, Digby said.

"We've lost Jimmy, a kākāpō we loved, and we've lost the opportunity for more genetic diversity."

Digby said Jimmy was one of his favourite kākāpō.

"I knew him well. He wasn't particularly sociable. He was one of the more wild birds, but was quite slow because of his arthritis.

"But he had a pretty good nature. They all have quite different personalities, and Jimmy was a good one."

We’re gutted. Jimmy the #kakapo has been found dead on Whenua Hou. Post-mortem has shown he died from complications from x-ray to investigate wing injury two weeks before. #conservation pic.twitter.com/OFTfNeP8mn — Dr Andrew Digby (@takapodigs) December 19, 2017

DOC sometimes struggled with deciding whether or not to get medical treatment for kākāpō or leave them in the wild, Digby said.

"It is tricky for us. We try to keep them as wild as possible and only remove them from the wild if there's a good reason to do so."

Digby expected the kākāpō population would decrease further before it increased.

"We're in a period where we won't have any significant breeding until early 2019. Kākāpō only breed every two to three years."

The kākāpō population did increase from 125 to 157 in 2016, however.

"We had 32 chicks produced last breeding season, which was massive. Every seven years we get a big breeding event, and 2019 could be one as well."

KEY KĀKĀPŌ FACTS:

- The bird freezes when disturbed instead of more useful forms of defence. This contributes to the decline in kākāpō population.