A Waikato wetland sanctuary is reintroducing rats to try to drive out birds fouling a prized lake.
The move is a last resort to try to scare off half a million sparrows and starlings that have found a safe spot to roost overnight around pest-free Lake Rotopiko.
They defecate so much they are changing the ecosystem and endangering the water quality.
Karen Denyer of the National Wetland Trust said 30 rats had been released to try to deter the birds.
If the birds shift where they sleep at night to outside the predator-free fence instead of inside, then they will poison the rats.
"It's quite a drastic approach," Denyer said.
"But we know we can get rid of the rats again - we've done it before.
"We want to see if this makes a difference. And if it does, maybe every so often we can put some rats in for a month or so."
Other anti-bird measures such as spraying an irritant in the air - "pepper spray", Denyer called it - have not worked.
A new science journal article out today that Denyer co-authored said "roosting exotic birds could potentially evolve as a major challenge for New Zealand as more sites become free of mammalian predators".
Rotopiko was especially vulnerable as it is small (10 hectares) and guano is a threat to peat lakes].
"It's actually burning off the native undergrowth and likely affecting the invertebrates, making it a rather unpleasant place to visit," Denyer said.
The numbers put pressure on native birds, too.
"I actually watched a falcon coming in to prey on sparrows, and a big mob of sparrows just balled up, surrounded it and pushed it right back over the fence.
"It was amazing to watch ... just the sheer volume of birds chasing it."
Other wildlife sanctuaries were also having similar, though lesser problems with birds coming in from outside to roost, Denyer said.
"I was in Riccarton Bush, which is also predator-fenced in Christchurch, just before Christmas, and there's certainly a build-up of feral pigeons in there.
"The birds are figuring out these places are the best best hotel in town."
They communicated the find to each other very quickly at Rotopiko, she added.
"So that's been quite a fascinating finding, just the extent to which birds will communicate and share that information. So the numbers built up very, very quickly."
The Rotopiko research used plates on a grid to gather guano and sound recorders to measure the cacophony of the roosting birds to estimate the numbers, because visual counts were not accurate enough.
RNZ
