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I found the same in this book, published just over 100 years ago. The book appears to be the result of some pretty rigorous study, so I expect it is a reliable source. On page 294, it refers to the gizzard as an involuntary sphincter, and again on page 302 it refers to the grinding of food in poultry as involuntary. In fact it says that because poultry ...
Involuntary. The gizzard, like our stomachs, works unconciously. However, chickens do choose to peck up stones to help along the process, whether that is by instinct or otherwise. (I raised chickens a long time but a proper reference is below) This book on commercial poultry raising says involuntary
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