Saturday, January 30, 2010
Cracking the Hard Shell
All along the coast, diving ducks are common during the winter: eiders, buffleheads, goldeneyes, scoters, mergansers, harlequins. They ride the waves, disappear beneath the surface, then pop back up like so many corks. Occasionally one comes to the surface with something in its bill, pauses a moment, swallows, then resumes doing its duck thing.
I watched a female Common Eider come to the surface with something that looked like a flattened egg. It was dark, appeared solid, and was about as wide and almost as long as her bill. Only later did I realize that she had brought a shellfish to the surface.
A lot of food is protected by a hard outer shell. Plants and animals have developed defenses against those things which would eat them. The hungry foragers in turn have developed ways to breach the defenses.
Let’s take a look at our bird feeders. If you do much bird feeding, you know that sunflower seeds are favored by many, perhaps most, of our feeder birds. The edible and nutritious part of the sunflower is inside its small, hard shell. Have you ever tried to open a sunflower seed? Putting aside the fact that we are big and clumsy when it comes to handling something that small, it is nevertheless a very difficult task to break open the seed.
Many birds have no trouble. The grosbeaks and cardinals can perch on the sunflower feeder or stand on the platform and shell half a dozen sunflowers seeds in the time it takes me to write this sentence. They have powerful beaks to crack apart the hard casing. Many smaller birds are equally adept. The finches, for example, will fill the nine perches on the sunflower feeder and consume seed after seed, scattering the husks to the ground beneath them.
There are a lot of different ways that birds have found to crack the hard shell of a nut. My favorite is the brainy solution used by native crows in New Zealand. They drop their favorite nut on a busy road, then wait for a car to run over the nut and crack it open. To avoid the danger of speeding automobiles, some crows drop the nut in a pedestrian crosswalk, and then wait for the light to change.
Omnivorous gulls occasionally exhibit an epicurean taste for mussels and other shellfish. They carry the tightly closed shellfish aloft, then drop it on rocks - or a roadway. They repeat this exercise until the shell finally opens, or breaks. Then they dine.
And yet, mussels are the favored food of diving ducks, and some dabbling ducks. The shellfish is swallowed whole and then the double stomach takes over. Complex contractions move the food back and forth between the glandular stomach (proventriculus) with its acids and enzymes which dissolve and digest, and the muscular stomach (gizzard) with its grit and stones which grind, pulverize, and mix. A study done on black ducks demonstrated that they can completely digest and pass a blue mussel in 30 - 40 minutes.
The hen eider which I watched with the large shellfish in her beak was unusual. Generally, they prefer smaller mussels which have less nutrition but which pose fewer problems when being swallowed.
Finally, I should add a disclaimer. Here and elsewhere I refer to the “tasty” meat, or morsel, inside the hard casing of a seed, nut, or shellfish. We may dine on mussels for their epicurean delight, perhaps their stimulus to the libido, and maybe their nourishment of the body. The hen eider which I watched was only concerned about the latter. Her complex and efficient digestive system quickly dissolves and grinds the hard shell and digests the meat. What taste buds she may have play no role in her menu selection. The diet of the eider has been determined by the remarkable and complex stomach which can digest a stone - or at least, food which looks like a stone on the outside.