This comes from Richard Feynman, who presented this in a talk at the fifteenth annual meeting of the National Science Teachers Association, in New York City (1966), titled “What is Science?”. It was later published in The Physics Teacher (1969).
During those walks in the woods, I learned a great deal. In the case of birds, for example, I already mentioned migration, but I will give you another example of birds in the woods. Instead of naming them, my father would say, “Look, notice that the bird is always pecking in its feathers. It pecks a lot in its feathers. Why do you think it pecks the feathers?”
I guessed it’s because the feathers are ruffled, and he’s trying to straighten them out. He said, “Okay, when would the feathers get ruffled, or how would they get ruffled?”
“When he flies. When he walks around, it’s okay; but when he flies it ruffles the feathers.”
Then he would say, “You would guess then when the bird just landed he would have to peck more at his feathers than after he has straightened them out and has just been walking around the ground for a while. Okay, let’s look.”
So we would look, and we would watch, and it turned out, as far as I could make out, that the bird pecked about as much and as often no matter how long he was walking an the ground and not just directly after flight.
So my guess was wrong, and I couldn’t guess the right reason. My father revealed the reason.
It is that the birds have lice. There is a little flake that comes off the feather, my father taught me, stuff that can be eaten, and the louse eats it. And then an the loose, there is a little bit of wax in the joints between the sections of the leg that oases out, and there is a mite that lives in there that can eat that wax. Now the mite has such a good source of food that it doesn’t digest it too well, so from the rear end there comes a liquid that has too much sugar, and in that sugar lives a tiny creature, etc.
The facts are not correct; the spirit is correct. First, I learned about parasitism, one on the other, on the other, on the other. Second, he went on to say that in the world whenever there is any source of something that could be eaten to make life go, some form of life finds a way to make use of that source; and that each little bit of left over stuff is eaten by something.