Audience effects, where individuals adjust their signaling behavior in the presence of bystanders, have been observed in several species. Research on the audience effect can provide valuable insights into the social cognitive abilities of animals because such signal adjustment likely represents strategic behavior by the signaler, based on an assessment of the impact of their signals on the bystanders. Especially, signal adjustment by two communicating individuals in response to the presence of third-party bystanders, observed as triadic audience effects, and flexible signal adjustment depending not only on the presence but also the composition of the bystanders, require more advanced social cognitive abilities. However, such phenomena have been scarcely studied. To test our hypothesis that large-billed crows (C. macrorhynchos), possessing advanced social cognitive abilities and complex social structures, strategically adjust their signaling behavior depending on the composition of bystanders, we investigated how the competence of bystanders as rivals affects allopreening interactions of male−female affiliative dyads. In mixed-sex flocks of subadult crows, male−female affiliative relationships are formed as a potential partnership of pair-bonds after sexual maturation. Given that mate quality has a great impact on reproductive success in the long-term social monogamous species, subadults are likely to guard preferred opposite-sex partners until pair formation. Thus, we predicted that affiliative subadult male−female dyads would display guarding behaviors, such as increasing allopreening to strengthen their partnership and to signal to same-sex bystander rivals. We further predicted that their guarding behavior would vary depending on the bystanders’ competence as rivals. Based on daily social interactions in a group-living environment, we determined affiliative male−female dyads as subject dyads, a rival female that exchanged allopreening frequently with the subject male, and a non-rival female that had no allopreening with the subject male. Each subject dyad was exposed to a rival female or a non-rival female as a bystander stimulus. The results showed that subject females demonstrated more frequent allopreening initiation and longer allopreening duration towards their affiliated partners in the rival condition compared to the non-rival condition, while males did not change their behavior. These findings indicate that large-billed crows flexibly and strategically adjust their signaling behavior in response to the social relationships between signal receivers and bystanders, which suggests that they have acquired the advanced social cognitive abilities necessary to succeed in intra-group competition within their complex social environment.