In the evening before they go to bed, the chickens will fill up on food so their crop, which is a pouch on the right front of their chest, will be full of food. If you look at them in profile they look like old dowagers with a full bosom. On the roost you can feel them and you'll be able to tell they stuffed themselves. Overnight their bodies process that food.
In the morning before they eat or drink, you can feel them again, and OH MY GOODNESS! they are completely flat in front! It's as if they have no crop at all. Don't worry, it's just a little bag for storing food. If it's flat and empty in the morning, that's a sign of good health. If there's still food in it, that may indicate a problem. Come in here and ask us sbout it, we'll walk you through it.
But a chicken that ALWAYS has an empty crop, now that's a problem! We want her full at night when she goes to roost. If that's not the case, we think she's not getting her fair share of groceries. You may need to feed her separately. Just give her her regular rations, but you may wet them if that helps her to clean up. She can have a little scratch, mealworms, fruits and veggies, but no more than 10% of her diet - a few tablespoons a day!
Hope this helps.