Birds DON'T perch on high-voltage wires

I was once on a nice slow narrow-boat trip between Devizes and Bath, and
several times we passed beneath high voltage wires and their associated
steel towers marching across the country. As I gazed at the wires (having
little else to do) I was struck by the fact that I never saw a bird sitting
on the high-voltage wires. There were often birds sitting on the tower's
highest wire, which is at earth potential, and which is there, I believe, to
help shield the main wires from lightning strikes. So, the birds were happy
to sit on a zero-voltage wire, but they didn't want to sit on one that has a
high-voltage on it.
Since then I've kept an eye out for birds on wires, and I have never seen a
bird perching on a high voltage.

You may or may not know that, if the voltage on a conductor is high enough,
it emits a silent current called corona. If there is a bump on the
conductor, the electric field there will be higher, and the corona will be
more intense. For example, when the first big Van de Graaff generator was
being built in a dirigible hanger in Round Hill, Massachusetts, one of the
graduate students' daily duties was to go out and shoot the pigeons in the
hangar. Pigeon poop, you see, constituted little bumps on the nice smooth
metal skin, and the bumps radiated corona. And this quite markedly reduced
the generator's maximum voltage.

My thinking is that a bird constitutes a bump on the wire conductor, and
this bump will radiate corona. Presumably, the bird feels this current, and
the feeling is unpleasant. He flies away. However, I've never seen a bird
land and then fly away. Perhaps they know before they even land that the
sensation will be unpleasant.

All this is speculation on my part. If it is true it might be an
interesting topic to investigate. For example, how high a voltage will the
bird accept before he flies away (or won't land)? Does it depend on the
bird species? Is the size of the bird a strong factor?

I done a hunt on the web, but I haven't been able to find anything relevant.
There are lots of sites that tell you why a bird CAN land on a high voltage
wire without electrocuting himself. The reason they give, of course, is
that the bird is not completing a circuit, and without a complete circuit
there is no path for the electricity to flow. Obviously, the writer had
never watched the birds; the discussion was completely theoretical.

Can any of you throw any light on this? I'm not a serious bird watcher.
I'm a physicist by training. I thought, however, that this observation
might be of more interest to a bird-watching group than to a physics group.

Regards,
Ron Verrall
Victoria, Canada


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Relevant Pages

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