Do they eat the shells?
Am guessing it's the diet.
With Scratch and Peck the can miss the vitamins/minerals/amino acids,
can be best to feed that feed wetted.
Key is heat stress in summer....I guess.....I dunno.
Maybe @ChickenCanoe knows if the scratch and peck web page that @GC-Raptor posted is valid.
IMO using ACV is a bit of a 'fad' and often mis- and/or over-used and over promoted.
Many do not understand that it's Active Cider Vinegar that can...