MY EDITORS WANTED a wildlife column, and I had a bazillion ideas, from tracking the yeti in Nepal to following a migration of white–eared antelopes in Sudan. They said, "No. How about your new backyard?"

So, in early spring, I began reappraising my adopted home city, the 300 square miles of concrete and steel that Kurt Vonnegut once called Skyscraper National Park. I quickly came to see that the metropolis, which claims to be world capital of everything from finance to sneaker collecting, was also one wild kingdom. Leslie Day's 2007 book Field Guide to the Natural World of New York City went so far as to say that New York is quite possibly the most biologically diverse big city in temperate America. This is no hollow boast. Suburban sprawl is dislocating wildlife, and NYC's healthy waterways, treed streets, 500 miles of coastline, and 30,000 acres of well-maintained parks are proving to be inviting habitat.

There are, of course, uncountable thousands of ordinary animals—bats, skunks, hipster goons on fixed–gear bikes—but there are also plenty of surprises. Peregrine falcons nest in the eaves of high rise buildings, supposedly more per square mile than almost anywhere else on earth. Harbor seals bask near the Coney Island boardwalk. Bears and moose have pawed at the city lines, coyotes and deer have wandered into Manhattan, dolphins and a Florida manatee have summered in the East River, and all sorts of reintroduced critters are doing well. Hedda Gobbler, a wild turkey that lives in Harlem's Morningside Park, is practically a tourist attraction.

Sounds exciting, but my first urban safaris were decidedly ho–hum. I failed at birding in Central Park (seeing nowhere near the 100 to 150 species that an expert can log) and grew bored with "mothing," hanging with three charming kooks who ogle drab lepidoptera in Central Park's Shakespeare Garden (with a black light—wow). Then I heard about José.

Three hundred years ago, New York was lousy with beavers. "You could walk across rivers without getting your feet wet," said Damian Griffin, education director for the Bronx River Alliance. But fur trappers wiped out the local population, and it wasn't until 2007 that an actual beaver was positively identified, after Bronx Zoo PR representative Stephen Sautner stumbled upon José's lodge, adjacent to the zoo's Bronx River parking lot. "It was in the zoo!" Sautner told me. "Twenty–five yards from where hundreds of animal experts park, and no one had seen it."

Sautner and staff named the beaver after the borough's green–minded congressman, José Serrano, and sent out a press release heralding the species's return as an emblem of New York's ecological health. Never mind that no one knew whether José's appearance was a freak accident or evidence of a returning, viable population—he made headlines. The New York Times: AFTER 200 YEARS, A BEAVER IS BACK IN NEW YORK CITY. New York Daily News: WELL, WE'LL BE DAMMED. Not long after, a flood destroyed the lodge and José went missing.

Last April, sweeping the East River around the UN building in preparation for Pope Benedict XVI's visit, the New York harbor patrol found a suspicious "dark foreign object" that turned out to be a beaver sick with pneumonia. The animal was raced north to a Castor canadensis specialist in Utica, but it was too late. Some feared the worst: that José had been flushed downstream, all the way to Midtown. But others continued to see gnawed stumps around the Botanical Garden, and some claimed to have actually seen José themselves. No one had photographic proof, though.

That was when I became obsessed with the elusive little fur ball.