The crop should be empty in the early morning. As they eat during the day, it should fill up by bedtime. When a crop stops working, it fills up and the contents stay put, not emptying overnight, which can eventually cause infection. Here are a few articles to read about crops:
http://www.the-chicken-chick.com/2012/04/answers-from-chicken-vet-on-impacted.html
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.chickenvet.co.uk/health-and-common-diseases/digestive-problems/index.aspx
http://www.the-chicken-chick.com/2012/04/answers-from-chicken-vet-on-impacted.html
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.chickenvet.co.uk/health-and-common-diseases/digestive-problems/index.aspx
