'Field Notes' Investigates Gizzard Grit

Jan 19, 2016

Afternoon sunshine was softening into twilight on a recent fall day as I drove with my family down a forest road in the mountains north of Missoula. We were heading home after a day of hiking and grouse hunting—and we had a blue grouse to roast for dinner. We rounded a bend to find a covey of seven ruffed grouse, milling about in the road and pecking at the gravelly surface. What were they doing?

While a bird’s beak is great for cutting, shredding, grabbing, or prying, it can’t compete with teeth for grinding food. Lacking teeth, birds process their meals in two stomachs: one glandular and one muscular. Food enters the glandular stomach first, where digestive juices begin to soften it. The food then moves to the muscular stomach, called the gizzard, which mechanically grinds it. Enhancing the gizzard’s millstone action are small bits of stone or grit that the birds swallow. The grouse we encountered that autumn evening were eating grit for their gizzards.

As we cleaned our intended supper grouse at home that evening, my two boys decided to see what this bird’s gizzard contained. We sliced it open and emptied the contents onto a sheet of freezer paper. The boys then carefully examined—and patiently counted—what they found. Inside were several partially digested huckleberries and grasshoppers, as well as 23 chokecherry seeds and a whopping 632 pieces of grit, ranging from about a quarter inch across to sand size.

I was surprised to see that almost all the grit was quartz. The bedrock where the grouse had lived was predominantly ancient mudstone sliced by some quartz veins and black igneous dikes. Why didn’t the gizzard’s contents reflect the diversity of the local geology? Was our sample of one an anomaly? Or had this grouse actually selected quartz grains from among the mix of rock types?

In a study of white-tailed ptarmigan in Colorado, researchers found that the birds in all their study areas preferred quartz stones as grit. Even in quartz-deficient areas, the birds selected for quartz. Why? I wondered. Maybe it’s because quartz is relatively hard. On the Mohs scale of mineral hardness, where diamond tops the 10-point scale, quartz scores a respectable 7 points. So, given that a grouse would be hard pressed to find forest roads surfaced with diamonds, quartz is a durable alternative. Or maybe it’s the color. We usually find grouse pecking for gizzard grit at dawn and dusk. Perhaps light-colored quartz is simply easy for the birds to pick out in dim light.

A Danish study of gizzard stones from a variety of birds found that some species select for stones that match the colors of their mates. That theory didn’t seem to apply here, but it lent credence to the idea that grouse might actively select their grit. A study of grouse and ptarmigan in interior Alaska found that grouse preferred lighter-colored gizzard grit than did ptarmigan, again implying some selective process.

But while some species of wild birds seem to exhibit preferences in gizzard grit, domestic turkeys appear, at least anecdotally, to be remarkably unselective. A woman—appropriately named Ruth Stone—who worked for 21 years as a gizzard cutter in a turkey processing plant, appeared on the David Letterman show to share what she’d found in turkey gizzards. Her treasures included marbles, arrowheads, an apothecary jar, a crucifix, coins, and—her favorite—a spark plug!

While I can only speculate why—or even whether—a bird selects the grit it swallows for its gizzard, I’ll be checking out the gizzard contents of any game birds we bring home in the future. Who knows what I might find?

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes," 11/29/15. Listen on air or online Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or via podcast.)