Multiple predators can additively or synergistically affect populations of shared prey. During the last 15 y, the Helgoland breeding population of the avian mussel predator S. molissima increased 10-fold, while blue mussel (Mytilus edulis) abundance decreased accordingly on the rocky intertidal shores of Helgoland. In a 16-month field experiment, we tested whether the combined predation impact of benthic and avian predators (= control) resulted in lower blue mussel survivorship than through the exclusion of avian predators alone and compared this to the overall predation effect (= complete exclusion cage). Predation effects were assessed for initially small (22 – 39 mm) and large M. edulis (40 – 60 mm), matching preferred and dismissed size ranges, respectively, of S. mollissima and the most abundant benthic predator, the green crab Carcinus maenas. Blue mussel survivorship was quantified in September 2014 and May 2015, i.e. just before crabs and common eiders generally disappear from and after they return to the intertidal, respectively, and, finally, again in September 2015. Survivorship of both, small and large M. edulis was not significantly different between control plots and plots where avian predators were excluded, but was in both treatments significantly lower than in plots where all predators were excluded with complete cages. This pattern was consistently apparent at each of the three sampling events. Our results suggest that the recent decline of the intertidal blue mussel population may have happened independently of the concomitant arrival of an avian mussel predator.