Are gizzard muscles voluntary or involuntary?

Question

I started raising chickens this year. They have amazing appetites, and I often wonder what eating is like for them. They have no teeth, so they don't chew their food as they eat it. They do seem to have some sense of taste, because they often bite off a bit of something and then spit it right back out. But the only equivalent of chewing for them occurs in their gizzard. Because the gizzard is pretty far down the digestive tract, I would assume that it's an involuntary muscle. But all chewing being unconscious for them also sounds kind of crazy.

I'm trying to discover the answer to this question on the internet and I'm surprised that the answer hasn't been easier to find. Does anyone here know? Do birds masticate unconsciously?

Answer 1

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

Answer 2

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 grind all of their food, whether it needs it or not, they found that chickens digest whole grain corn more completely than if the grain is first cracked or ground. That is in contrast to other farm animals studied. I also find that odd, because I have noticed that my hens strongly prefer cracked corn. I've often seen them pick up and cast aside whole or mostly whole kernels. I guess digestive efficiency is not their top priority.