– Aposematism is a defensive strategy that combines a deterrent with visual signals. High intra-species variation in learning time to avoid the signal is seen, and this variation in behaviour influences prey defense evolution. Here, we aim to investigate if the micro-gut biome is a driver in maintaining this heterogeneity. Research with mammals establishes a relationship between gut microbiome, diet and behavior. We ask if this relationship impacts ecosystems and test if there is a feedback loop between the predator gut microbiome, cognition, foraging decisions and concentration of prey toxins consumed, and how this influences the selection pressures on chemical defenses. This provides multiple objectives: First, testing the association between the predator gut microbiome and foraging behaviour, quantifying the effect of consuming aposematic prey on the predator gut microbiome, identifying the defensive toxins that impact the predator gut microbiome, measuring the causal effect of the gut microbiome on behavior and then testing the effect of the early-life gut microbiome on foraging behaviour in adulthood. The model predators employed will be wild-caught great tits (Parus major), that will undergo altered gut microbiomes, through prey toxins and faecal microbiota transplantations. The microbial gut composition will be measured, and behavioral experiments will assess avoidance learning and dietary awareness. This investigation will deepen our understanding of the role of the predator gut microbiome in aposematism and uncover insights into the variation in predator behavior. Additionally, this investigation can propose if the gut microbiome is a factor in shaping ecology and evolution.