You couldn't possibly know the answer without first knowing the type of rock and the species of the bird. Would a 50lb bag of basalt weigh the same as a 50lb bag of marble? Would a 50lb bag of black duck feathers weigh the same as a 50lb bag of goose feathers? Rhetorical questions of course, as we all know the answer to be no.
Let me ask you this. Are you planning on dropping these bags out of a three story window of a building located within a vaccume? If so, which do you hypothesize would hit the ground sooner? I would argue the rocks would, as the feathers would likely just fly away.