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Zoologist and Otago Museum honorary curator of entomology Anthony Harris said it was unusual to see cygnets there in midwinter.
It was possible warmer winters and climate change had contributed, and the past two years had been unusually warm on average, he said.
Birds New Zealand Otago representative Mary Thompson also said it was unusual to see cygnets there in winter.
In early June she had noted 100 adults and 50 cygnets.
A favourable food supply might have contributed to the breeding, she said.