In avian cooperative breeding systems, many benefi ts obtained by social pairs from the presence of helpers have been uncovered. However, until now, the factors that determine the type of assistance helpers provide and the responses of social pairs have not been well illustrated. We examined the contribution of helpers to cooperative groups and the relevant responses of dominant pairs in the azure-winged magpie Cyanapica cyana which breeds on the Tibetan Plateau. We used the capture -mark -recapture method to identify helpers. Results showed that helpers were mostly the yearling sons of dominant pairs. Th ey mainly contributed to the cooperative group in three ways, courtship-feeding the incubation female, provisioning the brood, and defending the nest. For responses of dominant pairs, we unexpectedly found that clutch size was not infl uenced by the presence of helpers at the nest. However, cooperative groups had higher brood feeding rates than biparental nests and their feeding pattern also diff ered to that of the latter. Consequently, nestlings in cooperative groups had larger fl edging body mass than that in biparental nests. By examining reasons for nest failure, we revealed that conspecifi c nest-raiding contributed to more nest failure than any other natural predators. Because of the contribution of helpers in defending against both predators and conspecifi c nest-raiders, cooperative groups had higher survival rate than biparental nests. Th us, our fi ndings suggest that in a highly-clumped nesting pattern, factors concerning the risk of nest predation, rather than that infl uencing food supply, play an important role in determining helper eff ects and responses of aided dominant pairs.

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