Coexistence conservation strategies: Testing conditioned food aversion to protect the endangered Western capercaillie

• Nest predation is a key limiting factor for capercaillie conservation. • Aversive-injected eggs were used in a BACI design to induce conditioned food aversion. • Treatment reduced red fox ( Vulpes Vulpes ) nest predation by 2.35 × in capercaillie habitats. • No significant effect was found on marten ( Martes martes ) predation. • Conditioned food aversion offers a non-lethal, ethical tool for predator–prey coexistence. Bird species, especially ground-nesting species, are experiencing sharp population declines. Nest predation is one of the main factors limiting the reproductive success of the capercaillie ( Tetrao urogallus ) in declining populations. This study represents the first trial assessing the use of Conditioned Food Aversion (CFA) as a non-lethal tool to reduce capercaillie nest predation, particularly by red foxes ( Vulpes vulpes ), within the framework of coexistence conservation strategies. The fungicide Thiram was used as an aversive compound in artificial eggs, and predation rates were compared between a treatment and a control area before and after conditioning (BACI design). In the treatment area, CFA produced a slight reduction in overall nest predation, whereas in the control area a marked increase was observed. At the species level, red fox predation in the treatment area was reduced by a factor of 2.35 compared to controls, demonstrating a significant effect of CFA. No significant effect was observed on predation by martens ( Martes martes ), suggesting lower conditioning effectiveness in mustelids. Furthermore, the presence of unconditioned juveniles and a slight delay in the post-conditioning phase may have influenced the results. Additionally, nest predation by wild boar ( Sus scrofa ) and brown bear ( Ursus arctos ) was detected at low to moderate levels. The study highlights the potential of behavior-based methodologies within coexistence conservation strategies, such as CFA, as viable alternative to lethal control. These tools are not only more socially and ethically accepted but also promote more effective and sustainable conservation of threatened species.