Is shared male assistance with antiparasitic nest defence costly in the polygynous great reed warbler?

► We explored whether great reed warbler social status affects antiparasitic defence. ► Monogamous nests were more male-protected than primary and secondary nests. ► Females of polygynous males did not compensate for lower male aggression. ► Nests of different status did not differ in susceptibility to brood parasitism. ► Shared male assistance with antiparasitic nest defence does not represent a cost.