If you asked one of the more than 20 million ''birders'' in the United States what he or she might regard as the ultimate bird-watching experience, the answer could well be: ''To go on a bird walk with Roger Tory Peterson as my personal guide.''
Since 1934, when the first edition of Mr. Peterson's ''A Field Guide to the Birds'' was published, launching a series of Peterson guides, millions of birders have spent blissful and sometimes exciting hours roaming field, forest, and shore with the guide in hand, binoculars hanging from the neck, and camera case slung over the shoulder.
Recently, a few fortunate Peterson admirers here enjoyed that ''ultimate'' experience of a walk with Roger Tory Peterson as part of the opening of ''Portrait of a Bird Watcher,'' an exhibition of the author-painter-environmentalist's works at the California Academy of Science in Golden Gate Park (through March 14, 1982).
On a foggy morning, a handful of serious bird watchers and about a dozen members of the press met with Mr. Peterson at the academy for a walk and a talk.
The guests would later have time to examine the Peterson paintings of eagles, owls, swallows, tanagers, and other American birds. But now they were knotted around a silver-maned figure who was responding patiently and at length to their avid questions.
A spare, fit man, Peterson speaks quietly but rapidly in a pleasant, carrying voice. He is asked about birds he hasn't seen but would like to. Bachman's warbler, he answers, which inhabits Southern swamps and winters in nothern Cuba, has so far eluded him. A wealth of detail on birds and other wildlife pours forth.
Over the last five decades he has written, edited, and contributed to scores of books on wildlife. He has taken thousands of photographs and produced many naturalistic paintings. His characterization of birds as ''ecological litmus paper'' led bird watcher Rachel Carlson to write the book (''Silent Spring'') that did much to launch the environmental movement.
Last year a completely new edition of ''A Field Guide to the Birds East of the Rockies - with new text, many color plates, and maps by his wife (Virginia Marie Peterson) was published. A labor of 10 years for the author, its issue was a major event for birders.
Peterson now is remaking his guide to the birds of Western North America, and his volumes on Mexico and Europe also will be revised.
The San Francisco party moves out of the exhibit hall into the ''Shakespeare Garden.'' Soon ''King Penguin'' - so nicknamed by his friends because the penguin is his favorite species - is busy identifying individual birds. Most he ''spots'' first by ear and then finds with binoculars. He says his eyes ''aren't so good now, but my hearing is sharp.'' Those around him, eagerly searching for the sparrows, warblers, thrushes, nuthatches, towhees, kinglets, and others he points to, remark that they would like to have the vision of this hawk-nosed, blue-eyed birder.
Peterson displays a couple of bird-watching tricks: Putting a fist to his lips, King Penguin sucks on it, producing a high, squealing sound. He explains that this makes adult birds think their nestlings are in distress and sometimes brings them close. ''Bostonians do it best,'' remarks the resident of Old Lyme, Conn., eyes twinkling as he glances at a member of the party recently arrived from the Massachusetts city.
''Voice'' is very important in identifying birds, he continues, noting that the advent of such aids as tape recordings (now available as an adjunct to Peterson guides) has been a boon to birders. Peterson laughs as he remarks that in trying to differentiate bird calls in print, one sometimes has to ''use weasel words.''
Coming upon a pond with several water species - from coots to swans - the bird man finds himself without a crumb to attract the flock. Deception again comes in handy: he bends over the water and moves his hand as though spreading food. Immediately the brood swims close. Fortunately, lest his avian beneficiaries lose faith in their most prominent advocate, a caretaker arrives with a bucket of cracked corn.
A horde of pigeons descends. ''They don't count,'' King Penguin remarks. He doesn't mean he dislikes pigeons, just that they are virtually a domesticated species.
Roger Tory Peterson is one of a dozen or so Americans who are to this culture what ''gray beards'' (elders) are or were to others. These are individuals who through insight, foresight, and experience, as well as their love of man and nature, lead others to seek solid values.
At this point in his life the world's leading bird watcher is receiving recognition for his contributions and achievements. But - just as he sturdily walks and talks and observes on this San Francisco outing - Peterson pushes on. He has years of work plotted ahead.
He tells the group that he sometimes resents the many hours spent painting and writing. ''I just enjoy looking at birds. But I have this Puritan conscience that compels me to make something worthwhile out of it.''
Speaking at the Earthcare Conference of the United Nations in June 1975 Peterson said: ''It is inevitable that the intelligent person who watches birds - or animals or fish or butterflies - becomes an environmentalist. . . .
''Of course, the ultimate observation is that man is the endangered species. A cliche, but true. The extinction of a species is a roadmarker of our decline. . . . If we create the physical, political, chemical, and moral environment that other forms of life cannot survive, by what insane arrogance do we think we could survive?''
Roger Tory Peterson is not a shouter. But his is a clear, firm voice urging each individual to ''define his own role in nature,'' thereby helping humankind to reach that point where it is ''enlightened enough to live with nature.''