Our Sebastopol, Loudmouth, has started shaking / vibrating his head when getting fed, if he sees a handful of greens, and when looking at the dog. I don't think hes scared because he walks right up to the dog when shes near the pen then does this while making goose noises and the dog runs off (shes a big wimp and the geese know it). I think its mostly when hes excited but not sure.