I believe they are also able to protect themselves in
another way, namely, by preventing the parasites from reaching their
bodies at all. I was out under the trees one day with a pet oven-bird
(Furnarius rufus), which had full liberty to range about at will, and
noticed that at short intervals it went through the motions of picking
something from its toes or legs, though I could see nothing on them. At
length I approached my eyes to within a few inches of the bird's feet,
and discovered that the large dry branch on which it stood was covered
with a multitude of parasites, all running rapidly about like foraging
ants, and whenever one came to the bird's feet it at once ran up the
leg. Every time this happened, so far as I could see, the bird felt it.
and quickly and deftly picked it off with the point of its bill. It
seemed very astonishing that the horny covering of the toes and legs
should be so exquisitely sensitive, for the insects are so small and
light that they cannot be felt on the hand, even when a score of them
are running over it; but the fact is as I have stated, and it is highly
probable, I think, that most wild birds keep themselves free from these
little torments in the same way.
Some observations of mine on a species of Orni-thomyia--a fly
parasitical on birds--might possibly be of use in considering the
question of the anomalous position in nature of insects possessing the
instincts and aptitudes of parasites, and organs manifestly modified to
suit a parasitical mode of life, yet compelled and able to exist free,
feeding, perhaps, on vegetable juices, or, like the ephemerae, on
nothing at all.
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