The beauty of sunflower seeds without the hulls is twofold. For birds, there is the benefit that none of their precious energy reserves during the cold winter months is wasted opening the shells. For humans, there is no mess of shells accumulating under the feeders that eventually will have to be raked up and disposed of. Since the shells of sunflower seeds contain a component that is toxic to grass and will kill any grass that is growing around or beneath the feeders, this is an important factor for many people in choosing whether to offer sunflower seeds in or out of the shell.