Rylee and Georgie visit the centre often and like the birds because "they're really nice and they're cuddly and soft and cute".
Their mother, Amy King, said she was unsure how the ornithological obsession had started. "They love visiting here and they know all the names of the native birds."
She said the girls had not hesitated when they heard the centre had been robbed and went "straight for their money boxes".
"They instantly said they wanted to give all the money they'd been saving. I think that's something kids are really good at - being selfless and thinking of other people."
Native Bird Recovery manager Robert Webb described the girls as "a hard-case pair".
"It's really neat," he said. "When you see the young ones getting behind something because they've seen there's a problem and they want to solve it, that's a good start in life."
The centre provides help, treatment and rehabilitation for injured and ill native birds so they can be returned to the wild. It also gives birdlife education to schools and visitors.
The centre, founded in 1992 by Mr Webb and his wife, Robyn, is run by a small band of volunteers.
See givealittle.co.nz/cause/helpnorthlandbirds to donate to the new mower.