Department of Conservation staff are working tirelessly to find out what has killed four kākāpō chicks and an adult bird this month.
The death of the four chicks including three within three days of each other leaves only 74 chicks in the breeding programme. There are 144 adult birds also in the programme.
Dr Andrew Digby tweeted that it was "troubled times for everyone involved as we try to understand what's caused the recent spate of aspergillosis in chicks and adults".
Tests have confirmed that several of the chicks have died from aspergillosis - a respiratory problem caused by fungus.
They have been carrying out CT scans at Auckland Zoo on some of the chicks and an adult kākāpō to help diagnose the cause.