Fantail population plummeting at predator-free Zealandia in Wellington

Fantail, wax-eye and grey warbler numbers are plummeting at the predator-free Zealandia eco-sanctuary in Wellington.

It's not necessarily a bad thing; the population of the birds which have been reintroduced are now booming, and are pushing out other species.

Before the fence around the Karori Reservoir Valley, the area was dominated by a few hardy native birds and common introduced species like blackbirds.

Te Papa curator Colin Miskelly says the endemic birds dominate the ecosystem.

"The introduced species and things like silver eye, fantail and grey warbler are pushed out to the margins."

Every single bird species which lived here before the fence was put up in 1999, native or not, has dramatically declined - except the tui.

Dr Danielle Shanahan, Zealandia Conservation manager says they have learnt they can create amazing, thriving bird communities in the middle of the city - but it's a little different to what they expected them to look like.

Native birds like tui and the tieke saddleback now dominate the valley.

"We are so fixated on predators being the problem of New Zealand ecology, and what this is showing is that once you get rid of those predators, it's competition between the species that makes a difference," says Mr Miskelly.

He says if places like Wellington are aiming to become predator-free, the make-up of native birds in our backyards will also change.

"Once you do, then everything changes - there are some species that do really, really well like a hihi, which is extinct on the mainland, but here in a little pocket in the middle of Wellington you can hear one of these deep, endemic New Zealand birds."