Wednesday, March 27, 2013

Don't envy the bird its flight

kw: book reviews, nonfiction, ornithology, natural history

Helen Macdonald wrote that, because a falcon has such "high-speed sensory and nervous systems ... their world moves about ten times faster than ours." I would turn that around. Events that seem very fast to us would seem much slower to a falcon. Very rapid senses characterize many birds. For example, our instruments show us that some bird songs which seem to us to contain just a few notes, really contain many very quick notes that we don't perceive, but other birds do. Where we hear a buzzy chirp, a fellow bird will hear a complex and perhaps beautiful melody.

Tim Birkhead has written Bird Sense: What It's Like to be a Bird to help us understand the sensory lives of birds. The speed of reaction noted above relates to at least sight and hearing, and possibly to touch. These are three of the seven sense areas that the book's seven chapters evoke: Sight, Hearing, Touch, Taste, Smell, Magnetism, and Emotions.

About sight, there is little controversy. It is well known that many birds' vision is much sharper than ours, while others see in smaller time slices, and that many see ultraviolet light, which we do not (Actually, someone who has had their lenses removed can see UV, which was filtered out by the yellow lens. Our blue receptor can see quite a bit deeper into the UV if given the chance. But birds have a fourth, UV-sensitive receptor that we don't have). Is any of these "better" than human sight? No bird has the full Monte. Each is fitted for the life it leads.

The chemical senses, taste and smell, have been much more troublesome. For one thing, they are harder to test for. To most people a bird's beak or bill is little more than a specialized fingernail sort of thing, and they do not consider whether it can feel or taste anything. Actually, it is full of nerves and several types of receptors, including taste buds, though in a bird, the majority of taste buds are farther back in the mouth. But how else will a bird know, when just the tip of its beak begins to pierce a bad-tasting caterpillar, that it is inedible? Yet it does know, and typically releases the insect before injuring it much.

What would it be like to directly sense the Earth's magnetic field? To always know where North is (unless you live near one of the magnetic poles)? Something the book doesn't bring out is that the magnetic vector changes its vertical angle as you approach or recede from the pole. I do recall reading about a study that showed how many migrating birds know by the magnetic dip that they are far enough north to land and nest.

I should like to have seen a chapter on the body-kinesthetic sense of birds, and how it compares with ours (this is sometimes called proprioception). Their physical reactions are so swift we can't see, for example, how an owl actually closes its attack on a mouse unless we film it at high speed.

All this high speed stuff got me to thinking. The speed of nerve impulses in humans ranges from rather slow to 100 m/s or more. In the brain, 40 m/s is more common. Our brain is larger than that of any bird. The longest axons in the human brain are about 16 cm. It stands to reason that a brain with fibers no longer than 1-2 cm can complete a thought about ten times as fast. Come to think of it, most birds and animals are smallish. The reflex arc between your fingertip and the ganglion in your spine that signals "Ouch! Pull away!!" totals more than half a meter in length. At 100 m/s, you can't possibly pull away in less than 8-10 ms, and most of us react in about 20 ms. A robin's longest reflex arc is no more than 0.15 m, so it could conceivably react to a nip on a toe or wing tip in 2 ms or less.

And back to the (possibly) beautiful melody. Do birds have an aesthetic sense? I have learned to turn anthropomorphism around. Of course they do. We have emotions because our vertebrate ancestors had emotions. If we can sense beauty, it must have preceded us. I suspect that any brain big enough to register pleasure and fear and anger can also register beauty. That would include all mammals and birds, and octopodes/octopuses (which are known to play), and perhaps squids and cuttlefish.

These are just a scant few items I found interesting. Bird Sense is fascinating, full of great anecdotes (Dr. Birkhead has been many places most of us have never heard of) and the results of an immense volume of research. I'd have gobbled up a book twice the size.

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