``It's a sort of . . . well . . . meditation on birdwatching,'' explains Nathan (Returning Your Call, 1975, etc.) to a friend interested in his latest writing venture. Well, yes, to put it mildly. Nathan is a poet of some repute, but let's be frank: Birds are what fire this guy's imagination. In spying them, he experiences their ``rare and real presence.'' Had he religious inclinations, he might have let their epiphanic qualities fashion him into a true believer. But he was not willing to surrender his sacred experiences with birds to the ether; he wanted to seize these epiphanies, take their measure. To that end he has collected, in brief concentrated episodes, a swarm of birdish thing: remembrances of delightful days afield with his bird-watching group, Thursday's Children; snippets of relevant bird poetry from Robert Frost to the Indian sage Valmiki to Walt Whitman; delicious tidbits, such as a description of the magnificent Aztec aviary the Spanish discovered when they reached Mexico City; the use of three field guides at once, ``enabling you to triangulate the bird, to come a little closer to its reality perhaps''; forays after tips received from the bird hotline; an ongoing disputation with his good friend Lewis, an ornithologist, about the exact meaning of his quest, an exchange that forces Nathan to get specific; and a superb telling of the apocryphal adventures of Virgilio Stampari, an imaginary 15th-century Italian explorer and collector of strange and wonderful bird lore. It is a mighty challenge, this effort to communicate with the ineffable, but Nathan never shirks. No smoky similitudes will do- -only luminous clarity. And while, like the furtive yellow rail, the big picture is elusive, the glimpses allowed Nathan are worth everything.