Hiyas,

Conditions like sour crop shouldn't really ever occur if the chickens are being fed properly, they need simple food like hard corn, wheat, barley, grit, layers pellets. Things that they can digest. If a lot of soft food is fed then the chicken will get bunged up and be unable to digest the food. Bread is one thing that is incredibly bad it goes to a goo that may then ferment. Likewise too much fruit or soft vegetables can cause it. As can worms or bacteria.

Ifd this is the same chicken that wouldn't perch last night I wonder if they are unwell and weak? How are they today.