After being successfully treated for lead poisoning and regaining his lost weight, male kakapo Lee, who was due to return to Codfish Island today, died suddenly at Auckland Zoo last night following a three-hour emergency operation.

A post-mortem examination carried out today by zoo vets has confirmed that Lee died from a small tear in the side of his crop - the sac-like part of the bird's digestive system where food is stored before it enters the stomach. This tear caused food to escape into the surrounding tissues, resulting in severe inflammation. Vets also discovered several areas of thinning in the crop wall, which they say could have predisposed Lee, an older bird of unknown age, to this type of injury.

"A tear in the crop is the equivalent of a stomach rupture in a human with the resulting, and often fatal, peritonitis," says the zoo's New Zealand Centre for Conservation Medicine (NZCCM) senior vet, Dr Richard Jakob-Hoff. "While working to repair this injury, Lee's heart stopped twice, but we managed to resuscitate him. However, he went into cardiac arrest again at the end of the procedure, and this time we were unable to bring him back - the shock on his system just too great." , He says that he and the rest of the vet team are devastated by Lee's death.

He says that up until Monday evening Lee, a founder kakapo who could be up to 100 years old, appeared to be in good shape, was behaving normally, and continuing to put on weight. However, by Tuesday morning he had deteriorated and had not digested any of the food from his last feed.

"When he arrived, Lee was emaciated, weighing just 1.2kg, and his survival was seriously in doubt, so gaining body weight was critical if he was going to pull through. Experience shows wild-caught adult kakapo do not readily adapt to eating in captivity, so tube feeding (using a special parrot formula) was the only way to provide adequate nutrition. Over a two-month period, Lee successfully put on half a kilogram, and was ready to be returned to Codfish Island. Tragically, we had no warning signs of this tear to his crop, as all his test results were healthy," says Dr Jakob-Hoff.

The Department of Conservation says crop feeding has been used with great success over the years to nurture kakapo back to health. DOC's technical supporter officer for the Kakapo Recovery Team, Daryl Eason, says since 2001, 21 kakapo have been crop fed and three were fed for periods of two and eight months. He says without such feeding, it is highly unlikely they would be alive today.

"Age seems to be the main difference between these latter three birds and Lee - they were all juveniles, whereas Lee was potentially a very old bird," says Mr Eason.

"This highlights the inevitable - that we're going to see an increase in age-related deaths in kakapo. Forty-six per cent of the current population are of unknown age (at least 27 years) - these are the founding birds transferred from Stewart Island to the safety of Codfish or Little Barrier islands in the 1980s. "Over the coming decades these birds will be reaching the end of their lifespan, so we expect to see the population stay relatively stable, as older birds die and are replaced with young. The health of the population will however, continue to improve as this age structure shifts," says Mr Eason.

"All of this reaffirms that the kakapo population is still very vulnerable, and that the intense management programme we are running is critical to the survival of this species."

Dr Jakob-Hoff says that the zoo's vet team has also been treating a female kakapo, Sarah, who was flown from Codfish Island to Auckland Zoo on Tuesday 21 October. Sarah, another founder kakapo from Stewart Island, is being treated for a wound to the side of her cloaca (possibly caused by injuring herself on a sharp stick) and subsequent weight loss.

"We had used a soft tube on Lee, but as a precaution for Sarah we will use an even softer tube. However, this is a fine balance, as too soft a tube can also cause problems by folding on to itself," says Dr Jakob-Hoff.