This is the line from a novel 'Baker's Blue-jay yarn' by Mark Twain. I know the meaning of both 'cover' and 'ground', but have no idea 'cover the ground'.
You may call a jay a bird. Well, so he is, in a measure - because he's got feather on him, and don't belong to church, perhaps, but otherwise he is just as much a human as you be. And I'll tell you for why. A jay's gifts, and instincts, and feelings, and interests, cover the whole ground.