A mystery illness has killed about 30 birds at the Waikanae River estuary, including two royal spoonbills.
About 25 black-backed gulls, two red-billed gulls, and a mallard duck have also been found dead at the Waikanae Estuary Scientific Reserve during the past 10 days.
"It is very unusual to have such a wide array of birds, in the numbers we have seen, dying in the same spot which feed on very different things," Kapiti SPCA manager Peter McCallum said.
He believed the deaths were probably caused by a toxic algal bloom or by bacteria in low water levels.
He was concerned about the deaths because people swam and walked their dogs in the area.
Wellington bird rehabilitator Craig Shepherd, who has been asked to treat a sick black swan found lying on the ground, said the symptoms suggested botulism might be to blame.
Botulism had not hit bird populations in the Wellington region before, but was not uncommon during periods of high temperatures further north.
He was feeding the black swan electrolytes every 90 minutes to help wash the bacteria out of its system.
The royal spoonbills have been sent to Massey University for tests to try to identify the illness.
Waikanae Estuary Bird Tours operator Mik Peryer said he had never seen anything like the number of recent deaths in his 20 years of work.
"People, including children, swim there ... it needs to be sorted quickly."
The SPCA had reported the deaths to Kapiti Coast District Council, the Department of Conservation and Greater Wellington Regional Council.
Mr Shepherd said it was very unusual for such a high number of birds to die in one area. He said the symptoms, including extreme weakness, inability to stand and muscle weakness suggested it could be botulism, another bacteria or poisoning.
Botulism had not hit bird populations in the Wellington region before, but the recent high temperatures could have meant the birds had ingested it from water or fed on birds affected by it, he said.
Treatment consisted of supportive care, fluid therapy - electrolytes every 90 minutes - to help wash the bacteria out of the bird's system.
The royal spoonbills have been sent to Massey University for tests to try and confirm the identify the illness.
- © Fairfax NZ News
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