Scientists know that monarch butterflies migrate thousands of miles each year between northern parts of the U.S. and Mexico or California, but no one has actually seen how they do it. So ecologist Pyle (Where Bigfoot Walks) decided to try. His method: to find individual butterflies at their northernmost habitat, follow them as far as possible, then repeat the process with other individual butterflies along the southward route. Amazingly, this haphazard approach worked. Pyle began near the Canadian border, at the Columbia River, and followed monarchs to the Mexican border--covering 9462 miles in 57 days and proving that western monarchs do not all migrate to California, as commonly believed. Though Pyle's account of his rambling trip suggests that much of it must have been more fun to live through than to read about, he enlivens uneventful sections with butterfly arcana, humorous reminiscences and rueful observations on the environmental impact of cattle ranching, pesticides, dams and jet skis. Pyle's laid-back humor is appealing, his descriptive talents are often poetic (he remembers monarchs pouring into a Mexican valley ""like a heavy orange vapor"" in which individuals resembled ""flecks of foam and water as they top a waterfall and plunge down into the foaming mass""). His memoir serves both as tribute to this majestic insect and as a thoughtful tour of the contemporary American West. Detailed sectional maps would have enhanced the book's appeal; endpaper map not seen by PW. (Aug.) FYI: Pyle is currently editing a collection of Vladimir Nabokov's butterfly writings.