Dead birds found after aerial poisoning operations targeting small mammal pests in New Zealand 2003-14

Abstract

In New Zealand, aerial poisoning with 1080 (sodium fluoroacetate) bait is widely used for control of introduced small mammal pests in remote or forested areas. However this practice is controversial, partly because of perceived risks to native fauna, particularly birds. That perception originally derives from substantial mortality of some native bird species in pre-1980 control operations, which prompted changes in baiting practice to mitigate most of the risk. However contemporary modifications to aerial baiting procedures - including sowing bait in strips or clusters, using prefeed, or adding deer repellent - may have renewed the risk to birds. Here we report observations on the number of dead birds found and on the relative abundance of certain bird species after 15 aerial baiting operations between 2003 and 2014. We found in total 81 bird carcasses, of which 84% were introduced species (mostly blackbirds (Turdus merula)) and 16% were native species (comprising eight kererū Hemiphaga novaeseelandiae, two tomtits Petroica macrocephala and one each of tūī Prosthemadera novaeseelandiae, fantail Rhipidura fuliginosa and silvereye Zosterops lateralis). Overall, significantly more bird carcasses were found per field day of searching effort after standard broadcast 1080 sowing than alternative bait distribution methods (strip- or cluster-sowing). Residues of 1080 were detected in 18% (2/11) of the native birds analysed (both tomtits), compared to 94% (33/35) of the introduced birds tested. Blackbirds comprised 80% of the introduced dead birds found, 96% of which (25/26) contained 1080 residue. Furthermore, in two detailed studies conducted in the Hauhungaroa Ranges in 2011 and 2013, blackbirds represented 3.2% and 1.9% of the overall live bird counts but 54% and 73% respectively of the dead birds found. Our results suggest that modern 1080 baiting operations pose only a negligible threat to native forest bird communities and a small threat to individuals, especially relative to the threats they face from the introduced mammals targeted in pest-control operations.