Ventriculus (Gizzard)
The ventriculus, or gizzard, is a part of the digestive tract of birds, reptiles, earthworms, and fish. Often referred to as the mechanical stomach, the gizzard is made up of two sets of strong muscles that act as the bird's teeth and has a thick lining that protects those...
As I understand it, some digestive juices are added in the mouth, but not much happens in the crop. From the crop food moves into the proventriculus where more digestive juices are added, then that moves into the gizzard where it gets all ground up.
Lots of details here...
Being the way I am, lol, I'm gonna take a look at the actual nutritional content of frozen peas. :D
I figure my hens and drakes eat so much other stuff while free ranging that the calcium in the layer feed is a non-issue. If they need a protein boost, I can always get a higher percentage layer.