My horses do just fine on nothing but grass, so there must be a good deal of nutritional value/calories in it. As humans we can't break down the cellulose in plants to get any calories out of it (so we call it fiber and it essentially "passes through us") so we think of it as low in calories. That doesn't mean there aren't plenty of calories there. The calories just aren't available to us. I can't speak to a chickens ability to break down cellulose, but I would imagine they're getting some calories out of there. Of course, they're probably also eating seeds, bugs, and worms when it looks like they're just eating grass. Those have plenty of protein and calories.