The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note of any observations you have made of the world around you. Rain, sun, wind...insects, birds, flowers...meteorites, rocks...seasonal changes...all are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.December 9, 2013
Gulls, like all birds, have no hands or fingers. Wings and flying are a higher priority. But pause a moment to think about all you do that requires use of your hands. And then imagine how difficult life would be without.
I watched a gull dispatching a chiton the other day along a quiet beach. It was cruising along in the ebbing shallow water looking for newly exposed prey, and spotted it, in spite of its seaweedy camouflage. Though gulls have no problem snatching up prey from shallow water, it's even simpler on shore.
Chitons provide a lot of calories for minimal energy expenditure. According to "optimal foraging theory" animals will deliberately choose foods with that in mind. Here is a long discussion of this concept, and this is a short description of optimal foraging theory in action, a study of Glaucous-Winged Gulls in Alaska. Chitons take less work to get into than barnacles and mussels, but they are only available at low tide. I see gulls out hunting invertebrates most often when the tide is going out.
Now, chitons don't put up much of a fight, being invertebrates (they are related to snails and clams, another type of mollusk). Their primary defense is attaching tightly to a hard substrate, with a row of eight overlapping shells embedded in a tough band of tissue providing structural support. So this Glaucous-Winged Gull didn't have to chase it down once it was levered off the rock, and it could drop it and pick it up again without fear it would scuttle away. Nevertheless, getting the edible part into a form to be swallowed is tricky.
Let's see how this encounter turned out...
(all photos in Lightbox; click for a larger view)
The gull steps out of the water, hooks the edge of the chiton, and variously maneuvers it...repositioning, crushing, sawing, pecking the softer parts.
With that beak and persistence, the gull prevailed. According to the time stamp on the photos, the whole sequence took less than a minute.
Among other feeding strategies, gulls can snatch prey from shallow water. Decorator Crabs are a favorite food, but they always stay below the surface. Even camouflaged, gulls will see them:
Gulls use an unusual and resourceful strategy for breaking into the more difficult shells of clams and snails: gravity. Roads along the shore around here are littered with shell fragments.
What's up in your part of the world? What have you been observing recently?
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