For years, the prairie around me was haunted by what I called ''invisible birds.'' In spring and summer, as I rode horseback across the prairie pastures, I could hear twittering close by, as though dozens of birds were singing on my shoulder or tiny bells were being rung close by. Quickly, I'd look overhead, expecting to see a flock of tiny singers. Nothing.
I'd look down, toward the curly buffalo grass that is the primary ingredient of beef raised here. I have ridden, driven, and walked over the same few acres for nearly 40 years. Simple repetition would have made me a good observer of prairie flora and fauna, even if I hadn't had an intense interest in everything that surrounds me.
Yet I could not see these birds. Even a very small bird ought to be visible in grass that is no more than six inches tall in a good year. If I couldn't see them, I couldn't identify them. If I couldn't identify them, I couldn't write about them.
Finally, out of simple frustration, I wrote about them anyway. A friend who knows the prairie intimately wrote to tell me they are horned larks, and I immediately looked them up in my nature guides. The photograph showed a bird larger than a sparrow, as much as eight inches long, its feathers shading from gray into tan -- perfect for blending in with the prairie grass.
In the photograph, the bird's throat is marked with a neat black crescent, and a black mask covers its eyes. On its head, another black patch marks its ''horns,'' feathers that can stand erect. (Pipits, a related species, lack the masks and the bow tie.) Its call is ''ti-ti'' .... Suddenly I started and looked out my truck's window. There it was, a horned lark. Tiny black feather horns stood up on his -- or her -- head; around its neck was a black crescent. The bird might as well have been carrying a sign.
I have always believed it is important for us to know as much as we can about the natural world in which we live, but this was uncanny. The experience seemed to prove my theory that knowing this bird's name had made it visible to me after 40 years of blindness. The bird pirouetted on the rock several times, making sure I got a good look at his markings, then announced -- ''ti-ti'' -- that he was going back to his breakfast, and darted up into the air.
Suddenly, I heard the high tinkling notes that had followed me for so many years. I stepped out of the truck and looked up; they were no longer invisible -- they swooped, darted, and zigzagged, obviously playing with the winter winds. They landed among the cattle, in a fluttering chaos of brown wings, and darted among the huge hoofs.
The cows grabbed mouthfuls of hay, shook their heads, snorted, and bawled at one another. Their footsteps shook the frozen ground. A big horned Hereford bull followed me to the dam and began drinking as soon as I'd chopped a hole in the ice. Beside him, a horned lark sipped at water I'd splashed on the ice. Every step, mine and the cows', was among the flock of birds, which were darting like quicksilver among the big legs, exclaiming as they gobbled the seeds that fell from the hay.
I explained to them how delighted I was to actually see them, but that they weren't being fair. If they'd hopped around on the ground under my feet 40 years ago, I would have seen them immediately. My accusations didn't bother them a bit. They sipped water, snatched hayseeds, and kept up their cheerful twittering.
Now that I have seen them once, I can always see them, and I marvel at how I could ever have missed them. Even sober reflection makes me believe that the difference was in knowledge. As with anything mysterious, as soon as I could name them, and believed in them, they could not stay hidden from me.
Horned larks, the shape-changers of the plains.