Comparison of populations that vary in cooperative behavior can point to factors that influence the evolution of cooperative breeding. In this paper we study cooperative territorial defense in two populations of the Australian magpie. These populations differed in social organization; in New Zealand group breeding was the norm and in Queensland simple pair breeding was the norm. We used caged territorial intruders to examine experimentally the participation of males, females, and juveniles in synchronous territorial defense. Within a population in Queensland and a population in New Zealand, adult males and females participated in cooperative defense significantly (p < 0.05 at least) more often together than alone. Between populations, Queensland pairs differed significantly (p < 0.01 at least) from New Zealand pairs in the level of response. As group size increased, percapita input in defense decreased in both populations. We discuss the importance of synchronous defense as a cooperative behavior in this species.