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Despite the unusual neural needs of birds, said Dr. Nottebohm, the fundamental premise could apply to all brains: to create good, strong memories that will last for months or years, they must devote whole neurons to the task, not merely stitch together a few synapses. "I think we're staring at a very basic principle here," he said. "The avian system may be a superb system to identify factors that are important for learning."

The implications of Dr. Nottebohm's ideas for humans are not entirely appealing. If he is right that the unit of memory is the entire neuron, rather than the synapse, then the aging brain with its neurons largely differentiated and locked into place looks considerably less flexible than the youthful brain, with its many unschooled neurons available to master novel skills. Humans can sprout new synaptic connections throughout life, but unlike chickadees they must largely make do with the neurons they were born with. If synaptic growth is in fact an inferior way to form memories and learn new tasks, then all those chirpy notions now in vogue that one can stay mentally fit by reading books, taking adult-education classes and keeping those dendritic trees ever-branching outward may be so much wistful thinking.

Instead, one's mental options and intellectual suppleness may start to shrink later in life as surely as does one's height.

"As you age, you probably have to manage to learn stuff in terms of the things you already know," Dr. Nottebohm said. "If you're good at metaphorical thinking, you can get away with people believing your mind is still young.

"But this may be why young people are so impatient with the wisdom of their elders," he added. "That wisdom has the flavor of things being interpreted with the ways of another era, which no longer seem relevant today."

Other neurobiologists praise Dr. Nottebohm for his elegant and often profound experiments, but they question how applicable his results are to species other than birds. They also insist that he must offer stronger proof that the turnover of neurons in a bird's brain is directly related to the animal's acquisition of a new skill or memory, rather than serving some other purpose, or no purpose at all.

"I've told Fernando that what he's doing is wonderful poetry," said Dr. Eric R. Kandel, a neurobiologist at the College of Physicians and Surgeons of Columbia University in New York. "There are lots of people who find that the world works exactly as everybody thinks it does, and then there are the very few who try to think about it in a completely different way. Fernando is one of the few."

Nevertheless, Dr. Kandel said: "I'm skeptical that cell turnover is used as a mechanism for memory, though I'm keeping an open mind. The ball is in his court now to prove it to the scientific community."

But neurobiologists confess that the scientific community's grasp of how learning and memory occur in any species is primitive and incomplete. For example, one of the favorite buzz phrases in neurobiology today is long-term potentiation: the idea that memories are laid down in the brain when a particular synaptic circuit is used enough times to become sensitized, and thus is far more likely to fire again in the future under the slightest stimulus. But trendy though the topic is, said Dr. Fred H. Gage, a neurobiologist at the University of California at San Diego, "there's very little evidence supporting the proposal that long-term potentiation is the correlate of learning and memory."

Apart from their poetic implications, the new experiments on birds are unusual for their reliance on free-ranging chickadees confronting the vagaries of nature, rather than lab-reared specimens incapable of finding seeds not presented to them on plastic dishes. The work was done at the Rockefeller University Field Research Center in Millbrook, N.Y., 1,200 acres of meadows, woods and marshes 80 miles north of Manhattan, where chickadees live in abundance. Studying the birds' habits, the scientists learned that in the fall the chickadees expand their home range from 3 to 30 acres and likewise expand their acquaintanceship with other chickadees, and that they begin storing their food.

For two years, the scientists captured 74 adult chickadees of both sexes and injected them with a radioactive tracer compound called thymidine. The compound is designed to be incorporated into any cells that divide, including brain cells. The researchers set the birds free again and recaptured 42 of them six or more weeks later.

For comparison, a group of chickadees living in an outdoor aviary and provisioned with food were also injected with the radioactive thymidine.

All the birds were eventually killed, their brains sliced into very thin sections and the sections scrutinized for evidence of thymidine, the signature of cell division. The scientists discovered that the greatest amount of new neuronal growth occurred in one region of the hippocampus, along the so-called ventricular wall. Because the size of the hippocampus had not increased, they assumed that old cells must have died off to make room.

They determined that cell turnover in the hippocampus continued throughout the year, but that it spiked most sharply in October, just as the conditions in the field were shifting most radically. At that point, almost 2 percent of the neurons were being replaced each day, an astonishingly high volume.

Among the aviary chickadees, the October hippocampal peak was significantly smaller.

Dr. Nottebohm points out that hippocampal damage has been linked to memory impairment in humans, monkeys and rodents, and he believes that the brain locus very likely plays a role in memory in chickadees. The temporal correlation between changes in the bird's hippocampus and changes in the world around it are too great, he said, to be coincidental. He next proposes comparing chickadees with other birds that do not store seeds, or with those who live in evergreen forests that do not change their appearance greatly each fall, to see if brain growth is meatiest in those who are neediest.

Photo: The black-capped chickadee. (Dr. Philippe Rousselot) Diagram: "Chickadee's Brain-Renewal Project" Chickadee's hippocampus grows many new neurons each year at seed-hiding time. New growth forms along ventricle walls (below right). (Sources: Dr. Philippe Rousselot/Rockefeller University) (pg. C14)