Many prey species mob predators to drive them away, thereby reducing their immediate and future predation risk. Given that mobbing is risky, it may also serve as an opportunity for males to advertise their phenotypic quality to females; however, this idea remains untested. We tested this hypothesis with a field experiment in south-eastern Brazil that assessed the response of sexually dimorphic bird species to models of two diurnal owls: a ferruginous pygmy owl (Glaucidium brasilianum), which mainly eats small birds, and a burrowing owl (Athene cunicularia), which mainly eats invertebrates and thus poses a low risk to birds. Across 19 bird species, the mobbing intensity was higher when facing the less-dangerous owl, and more males engaged in predator mobbing than females. The mobbing intensity of males was higher with a larger number of conspecific females present. This finding indicates that males may use mobbing to display their phenotypic quality to females, suggesting that predator mobbing may be influenced by sexual selection.