[Kuikentje]

My new girls eat lots of grit.  Presumably this doesn't just build up into a big pile in their crops - but it doesn't come out the other end either, at least not recognisably.  So what happens to it?  Does it get ground up in use and become undetectably small?
 

[8doubles]

The grit goes into the gizzard a sort of muscular stomach and grinds hard food such as seeds to make it more digestible.
The grit does come out as pebble like lumps after all the sharp edges have been worn off.
Sharp grit does a better job grinding than smooth pebbles which is why they need a good supply.
The gizzard lining can also be shed on occasion which looks nasty !

[Springlands]

When I was young I remember my dad dissecting a chicken gizzard for me so that I could see how it worked - it did smell quite a bit. That was back in the good old days when you had to clean out your own chickens before cooking. 

[Kuikentje]

The gizzard lining can also be shed on occasion which looks nasty !

Bleurgh!  Glad you warned me about that one!