West Auckland's latest clutch of precious kōkako chicks have fallen prey to rats after rodent numbers soared to record levels in the Waitākere Ranges.

Rat numbers exploded across New Zealand last year as residents in the Auckland suburb of Titirangi complained of huge "cat sized" rats brazenly scurrying through the centre of town.

Pest catchers also reported booming business as the rats ran riot across the city after feasting on food generated by a so-called mega-mast in which native trees fruited heavily.

Now conservationists say endangered kōkako chicks have paid the price with rats and other rodents reportedly destroying all the chicks and eggs in nests under observation at Forest & Bird's Ark in the Park conservation site in the Waitākere Ranges.

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"The loss of every single egg and chick we saw this breeding season is terribly sad for the kōkako," Forest & Bird's Annalily van den Broeke said.

"In the Ark, there are only about 60 kōkako and nationally about 3600 remain, so this is pretty heartbreaking."

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Rampant rats have been feasting on birds' eggs over the summer. Photo / Supplied
Rampant rats have been feasting on birds' eggs over the summer. Photo / Supplied

Conservationists said rat numbers doubled in the Ark conservation area between February and August 2019, with stoats and weasels also believed to have increased.

That led a team of three Forest & Bird staff and about 400 volunteers to ramp up efforts to stamp out rats from the 2270-hectare Ark conservation area.

The team with Auckland Council help laid 570 predator traps and 4800 rat bait stations.

Trees where kōkako nests were spotted were also surrounded by a "ring of steel", with extra traps laid in the area.

Despite the gauntlet of traps, predators were able to ravage three of the nests.

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"One nest had broken egg shells and rat droppings in it and none of the chicks we spotted successfully fledged," van den Broeke said.

Two kōkako chicks were banded, but then disappeared.

Huge rats took over Titirangi's streets last year. Photo / Supplied
Huge rats took over Titirangi's streets last year. Photo / Supplied

The team will do their next bird count in August and hope there are kōkako chicks that they didn't see over the summer, who managed to escape the rats and fledge.

"Most years, we find a few young birds from nests we didn't know about," van den Broeke said.

Kōkako vanished from the Waitakere Ranges in the 1930s and were reintroduced in 2009.

Predators attacked five out of eight kōkako nests monitored in the previous 2018 to 2019 breeding season. Nevertheless, four kōkako chicks fledged in the Ark and its buffer zone in early 2019.

Forest & Bird had warned heading into the summer that rampant rats posed a huge risk to kōkako.

Now they are asking Waitakere and city residents to pitch in with pest control efforts by putting traps or bait stations on their properties.

"The beautiful kōkako deserve every bit of help we can give them," van den Broeke said.

Experts said mega-masts typically occurred when the average summer temperature was more than a degree higher than the average temperature of the preceding summer.

Before pest predator species were introduced, such mast seasons offered native seed-eating birds a buffet to make up for lean years in between.

Ark in the Park was a pest control collaboration between Forest & Bird and about 400 volunteers, and landowner Auckland Council, in a 2270ha area in the Waitākere Ranges Regional Park.