Bioacoustic monitoring of lower North Island bird communities before and after aerial application of 1080

Modern aerial 1080 (sodium monofluoroacetate) operations are effective in reducing population densities of possums, rats and stoats, thereby reducing predation pressure on birds. Debate regarding 1080 use, however, continues to centre on potential non-target effects, with some groups claiming that birds are killed in large numbers such that forests “fall silent”. We investigated these claims by recording birdsong for 5–8 weeks before and after three separate 1080 operations in the Aorangi and Remutaka Ranges of the lower North Island, New Zealand. A Before-After/Control-Impact (BACI) design was employed for each operation, with simultaneous monitoring carried out in treated and untreated sites using autonomous recording units. The amount of birdsong was quantified, scoring the presence/absence of species’ calls in 30 ten second sub-samples for each of the 695 recordings analysed. The interaction between treatment and time and the “BACI contrast” was tested for at the community- and population-level. After each operation, the amount of silence was the same or lower in sites treated with 1080 relative to paired non-treated sites. Six taxa showed no evidence of an effect of 1080, while two of 26 taxa/treatment tests showed a significant interaction between treatment and time consistent with the silent forest hypothesis: chaffinch and tomtit calling rates declined following the Aorangi 2017 and Aorangi 2014 operations respectively. At p < 0.05, one or two “significant” results in 26 may be expected by chance. A negative impact on the introduced chaffinch (observed in one of three operations) is plausible considering the species’ granivorous diet. The pathway for a negative impact on insectivorous tomtits is less obvious; as a precautionary approach we recommend further research. Overall, we found little evidence of forests falling silent after aerial 1080 operations and the likelihood of negative population-level impacts via poisoning of native birds was concluded to be very low.