Abstract: Given the current pace of climate change, high temperature events will become increasingly frequent in many parts of the world. Predicting how high temperatures will impact the behavior of songbirds—highly sensitive to temperature change due to their tendency to be small in size, and to have high metabolic rates and diurnal habits—is therefore crucial and urgent. However, the behavioral impacts of high temperatures on acoustic communication in birds have rarely been studied. Responsiveness to antipredator signals is an important behavior to consider because failure to detect such signals could be lethal. We investigated whether, in great tits (Parus major), high temperatures would affect behavioral responses to playback of conspecific mobbing calls. We found a significant lag effect of temperature on how closely great tits would approach the playback speaker, with birds approaching less closely at higher temperatures. Further, we found that the emission of mobbing calls by great tits was affected by the current ambient temperature, with birds calling less at higher temperatures. The results suggest that at high temperatures, great tits change their tactic from active defense to less active response. High temperatures can thus induce behavioral shifts in great tits. In the current context of increasing average temperatures, such effect of temperature on response to vital indicators such as antipredator signals could impact survival when inducing greater risk of depredation. Significance statement: Climate change is causing heatwaves to increase in number and intensity. High temperatures can reduce the ability of birds to respond to vocalizations. Here, we test if high temperatures affect the ability of great tits (Parus major) to respond to conspecific mobbing calls—these calls generally serve to mob a predator and to recruit conspecifics and heterospecifics to join the caller. At higher temperatures, great tits produce fewer mobbing vocalizations and approach the loudspeaker broadcasting mobbing calls less often.