Cranial Kinesis

Often I watch birds just for general relaxation;  seeing them flit among branches or hop along the ground or tossing seeds in the air looking for the best parcel of food. But sometimes I focus intently and try to figure out what exactly a bird is doing, why it is doing it, and how its anatomy is constructed to allow it to accomplish that feat.

Ever watch a long-billed shorebird sticking its bill into the muck repeatedly and finally pulling up some morsel of food? (See video at the ABC.) Two things strike me about that. One is, how the heck to they find the food? They can’t see it. The nostrils are at the base of the bill and wouldn’t be able to detect scent underwater anyway. Can’t be sound. That leaves the sense of touch. And that’s how it’s done, but maybe not how you think.

Marbled Godwit

It would be very inefficient for a probing bird to probe repeatedly until it finally touched something edible. Random searching isn’t enough. The method is more refined. Sensory organs called Herbst’s corpuscles are located in pits at the tip of the bill with nerve endings going to the brain. The corpuscles sense pressure, so if pressure is greater to the left of the bill than the right, a potential prey item is to the left and the bird adjusts its probing.

Well, once the prey is located, and it could be several inches down in the muck, the next problem is grasping it. Opening up the entire bill just to grab the prey with the tip is not always easy to do because the mud and muck might be quite thick. So some birds, namely some long billed shorebirds, only open the tip of their bill. Think, for a second, about using chopsticks. You have to spread each of the chopsticks apart and then together to pick up a piece of sushi. You can’t just open the tips of the chopsticks.

Dunlin

The bird is able to do it because of a phenomenon called cranial kinesis. This is the term for the movement of bones in the skull relative to one another. Without getting into great detail here, look at the drawing of the bird skull. When the bird opens its jaw, the lower jaw goes down. As it does so, the quadrate bone, like a fulcrum, pushes against the jugal bar and pushes the upper jaw upward. No muscles are pulling the upper jaw up. This is how blackbirds, starlings, meadowlarks probe into the ground or into soft fruit and open their jaws to expose food items. And by an exaggerated method of cranial kinesis, some shorebirds are able to open only the tip of their bills.

Cranial kinesis is found in lots of animals, and is especially well developed in snakes, which can virtually dislocate their skull bones to swallow an egg or other food item bigger than their head. Ain’t evolution grand?

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