Just shooting from the hip, I would say that the vast majority of the birds (~95%) have a crop that is "full". I would guess that most of the empty or mostly empty crops come from morning roosters. Even then, there seems to be a little bit of weed seeds and some greenery. I have shot a couple birds that weren't healthy that had empty crops, but there are other telltale signs of their sickness that I wouldn't assume a bird wasn't good just because the crop was empty.
It sounds weird, but I really am fascinated and usually look at the crop contents if I see things beyond the typical corn/soybeans. As I mentioned, I see green leafy material quite a bit, even late into the year, so I know those birds are digging around looking for it. I also think its cool when some of the Kansas boys post up their crop pics and I see grasshoppers. SD grasshoppers don't make it too late into the fall.