“Very little,” admitted Lane. He knew how they nested—hollow stumps, mostly—and how they defended their nests against intruders. The last was hardly a pleasant subject.

“Come along,” said Professor Warren. She strode briskly downhill, speaking over her shoulder. “I’ve been doing some research on intrasensory substitution. Cases where one sense substitutes for another. Pit-vipers have a heat nerve in their foreheads so they can detect the most trivial of temperature variations, and so find warm-blooded prey in pitch darkness where their eyes can’t work. That’s heat perception instead of light. Bats feel obstacles with their ears. Buzzards have some superior substitute for smell. Put out a dead animal, even covered over with brushwood, or in a pit where it can’t be seen. Buzzards come from everywhere, immediately, even from upwind. They couldn’t possibly smell it upwind. And when they arrive, why then they try to find it with their noses! When the first buzzard comes downwind to bait that’s barely cold, he didn’t smell it! He saw the odor. It’s the only possible explanation. He simply has to be substituting some operation of his optic nerves for the sense of smell. You see?”

Lane hardly heard. Two miles back, something had tried to kill him, and his mind had not yet recovered its “balance. He’d seen nothing. It was impossible, yet it had happened.

“I was getting good results,” said Professor Warren vexedly, “but about ten days ago the buzzards went temperamental on me! Now they float up there, looking for food, and I put out bait which ten days ago they’d have flocked to. And they ignore it. It’s ridiculous! I’ve good proof that a good reek of organic decay can be detected optically. But I have to check through buzzards that it’s really done. And there are dead chickens in a barn yonder—” she waved a large hand—“and the buzzards aren’t interested! There’s a dead cow in a pasture, and they pay no attention! Temperament among buzzards? Or is it those damned dynamic systems I only halfway believe I’ve discovered?”