Unexpected lack of effect of the invasive american mink on the nesting sur-vival of forest birds

Nest predation by invasive mammalian predators can cause major impacts on native bird populations. The American Mink (Neovison vison) was recently introduced in Navarino Island, southern Chile, where it established itself as a new terrestrial mesopredator on the island, with documented impacts on waterfowls' breeding success. However, little is known about the effects of minks on forest birds' reproduction. Here, we investigated nest predation rates by native predators and the invasive mink on open-cup nesting forest birds by using artificial and natural nests. In six different plots, we deployed a grid of 14 (7 X 2) artificial nests spaced by 50 m and at random heights from the ground. We used camera traps in each nest to identify predators. At each plot, we estimated the relative abundance of predators using camera traps, Sherman traps, and bird point counts. We estimated nest survival probability as a function of nest age, concealment, distance to the river, and height. Additionally, we monitored 43 natural nests of five open-cup nesting bird species. Contrary to what was expected, the mink was not a main predator of the nests and preyed upon only one natural nest. The native raptor Chimango Caracara (Milvago chimango) was the main nest predator, preying on 39.8% of the artificial nests and 27% of the natural nests. We also found evidence that Chimango Caracaras learned to associate the artificial nests with the egg reward. We argue that the lower abundance of mink in the forest and a mismatch between mink peak activity patterns and bird breeding phenology can result in low depredation. Mink impacts may be more pervasive during summer months, on fledglings, or when mink activity peaks, and more research should be conducted to assess these ques-tions. Our results are valuable to better understand mink impacts on biodiversity and to prioritize conservation actions on species more se-verely affected.