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It is short fiction, not a novel, but just how short (novella, novelette, short fiction) I am not sure. I read it maybe four decades ago.

Humans have some (trading? scientific?) outpost, not a full colonisation, on a planet with two sentient native species, both very primitive, almost no technology. One has the ability to fly. I believe they are lighter-than-air gas balloons, but maybe this is a false memory, it is possible that they are just regular heavier-than-air, bird-like or bat-like or even large insect-like creatures. The other is a ground predator, that mostly eats small non-sentient animals. They do not normally attack live "flyers" (this term is mine, I don't think they are called "flyers" in the story). However, when one of the "flyers" dies from old age, or other natural causes, and falls to the ground, the predators (mostly pregnant females) do eat the corpses.

For some reason, some time after the humans arrive, the flyers develop a new habit. I think humans refer to it as the "New Tao", or "New Dao" of the flyers: when they realize they are soon to die, they fly away from the continent where the predators live to die on islands the predators cannot reach. The predators start noticing that many of their offspring are still-born, or die at birth. They accuse the flyers of stealing the souls of their offspring when flying away to die, and they begin to attack them whenever they come to the ground (which they must, to feed). To make a short story shorter, the humans understand that the bodies of the flyers concentrate some chemical element which is very rare on that planet, and necessary to the young predators. The pregnant female predators used to get enough of this element by eating the corpses of the flyers. Now that the flyers leave to die on the islands, the deficiency kills the predators offspring. The humans solve this problem by giving food supplements (the element is reasonably abundant on earth, so they can ship it in) to the predators so the flyers can go on with their New "Tao" or "Dao" without harm coming to the predators' offspring.

Hunter's Moon by Poul Anderson.

The flyers are the Ouranids and the land dwelling species the Dromids. The trace element that the Ouranids accumulate in their bodies is manganese. The new tao is:

"Oh…they’ve acquired a new—no, not a new religion. That implies a special compartment of life, doesn’t it? And ouranids don’t compartmentalize their lives. Call it a new Way, a new Tao. It involves eventually riding an east wind off across the ocean, to die in the Farside cold. Somehow, that’s transcendental. Please don’t ask me how, or why. Nor can I understand—or Jan—why the dromids consider this is such a terrible thing for the ouranids to do. I have some guesses, but they’re only guesses. She jokes that they’re born fanatics."

The story ends:

“I got the word from the Center,” she declared while she held his hand. “It’s definite. The chemistry was soon unraveled, given the extra clue we had from the reviving effect of blood.”


He turned about. “What?"


"Manganese deficiency,” she said. “A trace element in Medean biology, but vital, especially to dromids and their reproduction—and evidently to something else in ouranids, since they concentrate it to a high degree. Hansonia turns out to be poorly supplied with it. Ouranids, going west to die, were removing a significant percentage from the ecology. The answer is simple. We need not try to change the ouranid belief. Temporarily, we can have a manganese supplement made up and offer it to the dromids. In the long run, we can mine the ore where it’s plentiful and scatter it as a dust across the island. Your friends will live, Hugh.”


He was quiet for a time. Then—he could surprise her, this son of an outback miner—he said: “That’s terrific. The engineering solution. But the bitterness won’t go away overnight. We won’t see any quick happy ending. Maybe not you and me, either.” He seized her to him. “Damnation, though, let’s try!"

The story was originally published as part of the collection Medea: Harlan's World, which was an experiment in collaborative science fictional world-building, featuring contributions by Hal Clement, Frank Herbert, and others.