Catch of the Day: Bone-Crushing Gizzards

Arcata Wildlife Sanctuary
Arcata, California
December 2025

Klopp Pond at the Arcata Marsh is a favorite hangout for Greater Scaup. Since late summer, groups of males, females, and juveniles have been congregating there for the winter. Most often they are seen serenely floating across the pond like slow-motion ice skaters circling a rink.

But when hunger strikes, the scaup shift into action, diving below the surface and popping back up like corks with whatever food they find. To my surprise, I watched them surface holding muddy clams in their beaks—and then swallow them whole. How does a bird digest such a large hunk of hardened calcium carbonate?

I’ve seen many birds—and otters—labor to crush a mollusk’s shell or pry open its tight seal in order to reach the soft, nutritious body inside. But these birds employ a very different strategy. They consume the entire animal, shell and all. The clams have no defense against this approach.

Once swallowed, the shell is bathed in strong stomach acids that help begin the breakdown process. The scaup’s gizzard—armed with exceptionally powerful muscles—then pulverizes the shell into tiny fragments. These fragments are eventually expelled, much like an owl pellet. The birds also extract some nutrients from the shell itself.

The gizzard, as you may recall, is a specialized part of a bird’s stomach that contains grit and small stones the bird has ingested, called gastroliths. In the absence of teeth, the gizzard acts as a grinding mill. While I knew this in theory, I hadn’t realized it was capable of demolishing an entire clam nearly as wide as the bird’s neck. The full digestive process takes roughly 30–40 minutes.

Watching the scaup feed was a reminder of the quiet ingenuity of evolution—of bodies shaped precisely for their place in the world. What appears gentle and graceful on the surface conceals an inner machinery of astonishing strength.

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