On our early morning walks with Kodi, Srini often asks a question, the answer to which I'd forgotten (which happens a lot). This leads to a bit of research and sometimes a blog post. Like most mornings, today we walked into the hayfield along Bald Hill Road. Kodi leads us there and then he moves slowly, tail down and ears back, as he sniffs each clump of little bluestem grass. Kodi carefully studies the scent of wild turkeys or more likely the coyotes that are tracking the birds. The thought of wild turkeys led Srini to ask about a bird's crop and whether that helps to grind up acorns that turkeys devour in the fall.
I had to refresh my understanding of a bird's digestive system. A sketch by David Allen Sibley in National Audubon Society's The Sibley Guide to Bird Life & Behavior helped:
Many birds, but especially pigeons, doves, and "gallinaceous" (turkeys for example) have large crops. A crop is an expandable pouch that is an extension of the esophagus. It serves as a storage chamber, much like a chipmunk's cheek pouches. Birds stuff extra food into the crop where it can be digested later from the safety of cover. A bird with a full crop may look like it has a huge Adam's Apple--a large protrusion at its throat.
Food stored in the crop eventually makes it way into a bird's two-chambered stomach. First into the proventriculus where digestive juices begin to soften up the food, and then into the gizzard. It is the strong, muscular gizzard with rough skin-like ridges and an accumulation of grit that finally grinds the food--such as acorns--into smaller bits. The gizzard in birds acts like the grinding molars in mammals or similar to a millstone grinding wheat into flour.
Pigeons and doves have a particularly unique crop. When pigeons are born they reach into their parents mouths for "crop milk." This fluid, rich in protein and fat, is produced in the crop of both parents by a sloughing of fluid-filled cells from the crop lining. Young pigeons rely on this milk for their first 5 to 10 days, before switching to regurgitated plant and insect matter.
Bird digestion is actually quite efficient, so food stored in the crop doesn't last long.