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.