Anticoagulant rodenticide brodifacoum detected in dead nestlings of an insectivorous passerine

The anticoagulant rodenticide brodifacoum is widely used to eradicate invasive rats from islands for the protection and restoration of populations of native species. However, brodifacoum is also highly toxic to birds. We report the first apparent case of secondary brodifacoum exposure and subsequent poisoning in nestlings of an insectivorous passerine, the Stewart Island robin (Petroica australis rakiura). Thirteen dead nestlings were collected 3–4 months after brodifacoum bait was applied to eradicate rats from Ulva Island, New Zealand. Twelve of these composite nestling samples contained moderate concentrations of brodifacoum (mean = 0.08 ± 0.02 µg g^–1; range = 0.011–0.28 µg g^–1) at levels comparable with those associated with mortality in adult birds of other species (0.2 µg g^–1 in liver), which suggests exposure in the robin nestlings was lethal. However, we were unable to determine the definitive cause(s) of death because sampling of dead nestlings was opportunistic and we were unable to test live nestlings for residue as a comparison. The period between brodifacoum application and mortality in the nestlings (52–92 days) implicates secondary poisoning. Our results highlight the potential role of invertebrates as vectors of anticoagulant rodenticides in the environment, as well as the need for further research on this exposure pathway.