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Mixing sugar and water together may not seem a recipe for disaster, but left for several days could be deadly for unsuspecting nectar-feeding tui and bellbirds, Russell Evans, of Invercargill, said.
"We could be killing them through our kindness."
Mr Evans and his wife, May, have noticed fewer native birds around their bush-clad Otatara home this winter and, after witnessing a tui gasp for air, the couple knew something was amiss.
The couple sent two tui specimens to Massey University for an autopsy, which revealed the birds tested positive for aspergillus, a respiratory disease caused by a fungus infection.
Massey University avian and wildlife health senior lecturer Brett Gartrell said while it was unlikely the tui contracted aspergillus from dirty sugar water, unclean practices would result in the death of birds.
Salmonella and candidiasis - the same bug which causes thrush in humans, were deadly to birds and could lurk in unchanged sugar water unless containers were emptied and cleaned, he said.
Mr Evans said he did not want to discourage people from putting out sugar water for birds - a vital source of nourishment during winter, but urged people to put out just enough food to last a day and then when finished wash and sterilise the container.
"It just takes one person not to do this and we can have a lot of sick birds on our hands. Once you see a bird gasping for air, it is basically curtains for it."
Department of Conservation scientific officer Ralph Powlesland, of Wellington, said sugar water was an essential food source during the winter months for nectar-feeding birds such as tui and bellbirds.
It was important people continue with the practice of putting out sugar water in colder southern areas as there were fewer nectar sources available to birds.
"It doesn't matter what sort of container it is in, as long as it is a clean container."
How to feed nectar-feeding birds sugar water. -
> Dilute one cup of sugar with two litres of water in a clean container
> Put out just enough sugar water to last for a day
> Wash the container each day
> Sit back and wait for the birds to arrive