Why native birds are starving

The birds of the Nelson Lakes National Park are starving, says a conservationist.

Bryce Buckland said that for most of the year the bird feeder in his St Arnaud backyard was quiet, attracting a couple of tui or bellbirds each day.

In late summer, however, dozens of native birds would wait on a fence or in the trees for him to fill the feeder, ready to swoop.

‘‘I fill up that 3-litre container probably three times a day. Often the tui and bellbirds are so hungry they just fight like cats, they’re just in there bloody scrumming for it. They’ll empty that 3-litre container in 15 or 20 minutes.’’

The change in the birds’ behaviour, said Buckland, corresponded with peak wasp numbers in the native beech forest around Lake Rotoiti.

The common wasps eat the birds’ most valuable food sources – honeydew and insects – over summer, driving them out of the forest to Buckland’s backyard.

‘‘Years ago, before we had wasps, this area was just bursting with birdsong. All the birds had plenty to eat, but wasps took care of that,’’ he said. ‘‘The birds get really hungry.’’ Buckland has been involved in wasp control around Nelson Lakes for about 16 years. A founding member of conservation group Friends of Rotoiti, he sees the wildlife change each summer in the presence of wasps.

His story about the bird feeder is a crude measure of the impact wasps are having on native birds. But his theory is supported by sound science.

A 30-year study of native birds in Nelson Lakes National Park, published in 2010, found five species have declined in abundance – the bellbird, rifleman, grey warbler, New Zealand tomtit and tui.

‘‘We suggest that invasive alien species are the most likely cause of the ongoing declines in common native species,’’ the paper says.

It identifies wasps and possums as ‘‘two large changes in biology’’ in the national park during the study and speculates that they might be the cause of declining bird numbers.

Common wasps arrived in the Nelson region in the 1980s. Each summer, when they are most active, they clean out huge amounts of food that was once available for birds.

New Zealand’s native birds have evolved with honeydew, a nutrient-rich carbohydrate.

But common wasps raid masses of honeydew during summer.

A 2001 study by Dr Jacqueline Beggs found that wasps reduced honeydew supplies by more than 90 per cent for five months of the year.

Wasps also consumed up to 8.1 kilograms of insects per hectare of beech forest each summer – similar to the amount consumed by insect-eating birds.

There are about a million hectares of beech forest in the South Island.

Beggs’ study found that wasps ‘‘probably reduce or eradicate’’ some insects, removing another vital food source for birds.

A 1991 study found that there was ‘‘considerable dietary overlap’’ between wasps and native birds.

The kaka was also threatened by wasps. There have been concerns that the scarcity of honeydew was limiting their breeding.

Buckland said developments in wasp control technology had seen birdlife bounce back in recent years.

The protein-based bait, Vespex, developed by Nelson insect ecologist Richard Toft, had been remarkably successful, he said.

Deployed in late summer once the wasps have switched to a protein diet, Vespex could knock out entire nests overnight.