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Noisy silence

New Zealand’s use of toxic aerial baits to rid the islands of invasive mammal predators has attracted claims that forests “fall silent”, with birdsong declines after each operation. 

Listening to the forests, scientists have found little evidence for the claims, according to a study published in the New Zealand Journal of Ecology. At a species level, however, chaffinches (Fringilla coelebs) and tomtits (Petroica macrocephala) did make less noise following single bait applications, suggesting they may have been exposed to the poison.

For chaffinches, Roald Bomans and colleagues from Victoria University of Wellington (Te Herenga Waka) say this is a plausible conclusion as the birds are granivorous and thus likely to eat the toxin (sodium monofluoroacetate, otherwise known as 1080), which is delivered in cereal pellets. 

For tomtits they conclude it’s less likely in the context of other studies but recommend further monitoring. In fact, evidence suggests this resilient species has benefited from 1080 operations with higher reproductive success following predator declines.

For centuries, introduced rats, weasels, ferrets, possums and stoats have annihilated large swaths of New Zealand’s unique bird populations, consumed native vegetation and spread disease, threatening biodiversity, agriculture and precious cultural icons such as the flightless kiwi.

Six decades ago, the community united to remove the pests from their islands, known in Māori as Aotearoa, and now aim for total eradication with the “Predator Free 2050” initiative using a combination of measures including sanctuaries, trapping and hunting.

More controversial is aerial dropping of 1080, with some groups worried that it kills the very birds it aims to protect. Indeed, since the method’s inception in 1956, birds from 19 different species have been found dead after the poison was dropped. But most of these occurred before 1980, when carrot baits were used to deliver it.

Since the bait delivery was shifted to cereal grains and non-toxic pre-feeds were introduced, they posed significantly reduced risk to bird species, according to the group from Te Herenga Waka.

“Nonetheless,” they write, “further changes to baiting practices call for continued vigilance with respect to non-target impacts.”

To investigate, they used bioacoustic monitoring before and after three different 1080 drops in the Aorangi and Southern Remutaka Ranges of the lower North Island in 2014 and 2017, and non-treatment comparison sites in the Taraura and Northern Remutaka Ranges.

Birdsong increased slightly overall after the 2014 Aorangi drop and remained virtually the same in non-treatment sites during the same period. In 2017, birdsong dropped slightly in both sites and was not associated with 1080 drops.

After the 2017 operation in Southern Remutaka, birdsong increased in treatment sites 2–6 weeks after the drop, and decreased in non-treatment sites. In all cases, the team says the increases and decreases were minor.

Of the nine native species studied specifically, tūī (Prosthemadera novaeseelandiae), silvereye (Zosterops lateralis), rifleman (Acanthisitta chloris), grey warbler (Gerygone igata) and fantails (Rhipidura fuliginosa) showed no impact and three others in fact made more noise. 

The tomtit showed an increase and decrease in two of the operations and the chaffinch, an introduced species, showed a slight decline after only one operation.

The results support previous data that found “negligible threat to native forest bird communities” from current baits, according to Boman and team.

“We know from previous work that most native New Zealand forest birds benefit in the years immediately following effective mammal control,” says senior author Stephen Hartley. “This study confirmed that modern 1080 operations do not cause forests to go silent, and that few, if any, native birds are suffering short-term adverse effects.

“Regrettably, without appropriate control of introduced mammals, population declines and extinctions of Aotearoa’s native and unique biodiversity will continue.”