Birds often aggressively defend their territories against competitors of their own species as well as birds of other species. For example, bull-headed shrikes (Lanius bucephalus) are known to behave very aggressively against individuals of other species, but the reasons for these high rates of interspecific aggression remain unclear. I investigated aggressiveness of bull-headed shrikes towards other species in relation to four factors: (1) body size of subject, (2) distance between shrikes and interspecific individuals, (3) interspecific individual’s behavior, and (4) location of interspecific individuals within the shrike’s territory. Body size of interspecific individuals was the only significant factor that affected the strength of the shrike’s aggressive responses. Shrikes behaved more aggressively towards species with larger body sizes than they did to species with smaller body sizes. The reason for this might be that bull-headed skrikes sometimes prey on smaller birds. Therefore, smaller species perceive shrikes as a predation threat and flee them as they approach. Conversely, larger bird species are not threatened by shrikes and so shrikes need to have a more aggressive response to these larger birds in order to drive them from their territories.