The unwelcome visitors

The Rotopiko peat wetlands are a haven for rare and threatened wildlife. But when a flock of introduced birds numbering in the hundreds of thousands moves in - threatening the very nature of this special place - a group of people come up with some crazy-but-genius ideas to protect the wetlands and wildlife. Claire Concannon investigates.

Half an hour south of Kirikiriroa Hamilton, the Rotopiko wetland reserve is an oasis of trees in a desert of farmland.

At dusk, these trees fill with hundreds of thousands of starlings and sparrows, jostling for position on the branches, like noisy restless shadows.

It's an incredible spectacle... but these birds are not welcome here.

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"These birds are bringing in massive amounts of nutrients," says Karen Denyer, the executive officer of the National Wetland Trust of New Zealand.

"'We are trying to restore this wonderful site... and we've got this unintentional and kind of unexpected problem."

Rotopiko is a peat lake wetland complex. An ecosystem naturally low in nutrients, to which some rare native New Zealand species are perfectly adapted. For example, the endemic giant cane rush, and the caterpillar that lives inside it. They were translocated into the sanctuary years ago, as the group set about restoring the area.

It's been a lot of work - a massive weeding effort, erecting a predator-proof fence, removing predators, building paths and a boardwalk, creating an information trail, replanting kahikatea trees.

But now it's at risk.

Just two years after the predator-proof fence was put up, and the predators removed, birds started to come to roost at night inside the ten-hectare fenced area in large numbers. The nutrients in their droppings threaten to change the very nature of the wetland.

Because there are so many birds it's a major challenge to count them. Researchers from Toi Ohomai Te Pūkenga and Wintec Te Pūkenga have come on board to help. They have developed different methods to estimate changes in the number of birds, so the Wetland Trust can test the effectiveness of different deterrents.

Laser lights, noise, shooting, a product like pepper spray for birds, wood vinegar - nothing has worked so far. This has forced the Wetland Trust into trying a less desirable and more controversial method - returning rats to a rat-free area.

Listen to the episode to learn more about the Rotopiko peat lake complex, the flora and fauna that live there, and to hear the incredible sound of hundreds of thousands of roosting birds.

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Go to this episode on rnz.co.nz for more details