Perhaps no living scientist is as enthusiastic — or doctrinaire — a champion of Darwinian sexual selection as Richard Prum, an evolutionary ornithologist at Yale University. In May 2017, he published a book, “The Evolution of Beauty,” that lucidly and passionately explains his personal theory of aesthetic evolution. It was nominated for the Pulitzer Prize for general nonfiction, but within the scientific community, Prum’s ideas have not been as warmly received. Again and again, he told me, he has asked other researchers for feedback and received either excuses of busyness or no reply at all. Some have been openly critical. In an academic review of Prum’s book, Gerald Borgia, one of the world’s foremost experts on bowerbirds, and the ethologist Gregory Ball described the historical sections as “revisionist” and said Prum failed to advance a credible case for his thesis. Once, over a lunch of burritos, Prum explained his theory to a visiting colleague, who pronounced it “nihilism.”

Last April, Prum and I drove 20 miles east of New Haven to Hammonasset Beach State Park, a 900-acre patchwork of shoreline, marsh, woodland and meadow on Long Island Sound, with the hope of finding a hooded warbler. Birders had recently seen the small but striking migratory species in the area. Before he even parked, Prum was calling out the names of birds he glimpsed or heard through the car window: osprey, purple martin, red-winged blackbird. I asked him how he was able to recognize birds so quickly and, sometimes, at such a great distance. He said it was just as effortless as recognizing a portrait of Abraham Lincoln. In Prum’s mind, every bird is famous.

Binoculars in hand, we walked along the park’s winding trails, slowly making our way toward a large stand of trees. Prum wore jeans, a quilted jacket and a beige hat. His thick eyebrows, round spectacles and sprays of white and gray hair give his face a vaguely owlish appearance. In the course of the day, we would see grazing mallards with emerald heads, tree swallows with iridescent turquoise capes and several sparrow species, each distinguished by a unique ornament: swoops of yellow around the eye, a delicate pink beak, a copper crown. On a wooded path, we encountered a lively bird flinging leaf litter into the air. Prum was immediately transfixed. This was a brown thrasher, he told me, describing its attributes with a mix of precision and fondness — “rufous brown, speckled on the breast, yellow eye, curved beak, long tail.” Then he reprimanded me for trying to take a picture instead of observing with my “binos.”

About two hours into our walk, Prum, who is a fast and fluid talker, interrupted himself midsentence: “Right there! Right there!” he said. “There’s the hooded! Right up against the tree!” Something gold flashed across the path. I raised my binoculars to my eyes and scanned the branches to our right. When I found him, I gasped. He was almost mythological in his beauty: moss-green wings, a luminescent yellow body and face and a perfectly tailored black hood that made his countenance even brighter by contrast. For several minutes we stood and watched the bird as it hopped about, occasionally fanning white tail feathers in our direction. Eventually he flew off. I told Prum how thrilling it was to see such a creature up close. “That’s it,” Prum said. “That moment is what bird-watching is about.”

As a child growing up in a small rural town in southern Vermont, Prum was, in his words, “amorphously nerdy” — keen on reading and memorizing stats from “The Guinness Book of World Records” but not obsessed with anything in particular. Then, in fourth grade, he got glasses. The world came into focus. He chanced upon a field guide to birds in a bookstore, which encouraged him to get outdoors. Soon he was birding in the ample fields and woods around his home. He wore the grooves off two records of bird calls. He befriended local naturalists, routinely going on outings with a group of mostly middle-aged women (conveniently, they had driver’s licenses). By the time Prum was in seventh grade, he was leading bird walks at the local state park.

In college, Prum wasted no time in availing himself of Harvard University’s substantial ornithological resources. The first week of his freshman year, he got a set of keys to the Museum of Comparative Zoology, home to the largest university-based ornithological collection in the world, which today has nearly 400,000 bird specimens. “I’ve been associated with a world-class collection of birds every moment of my adult life,” he says. “I joke with my students — and it’s really true — I have to have at least 100,000 dead birds across the hallway to function intellectually.” (He is now the head curator of vertebrate zoology at Yale’s Peabody Museum of Natural History.) He wrote a senior thesis on the phylogeny and biogeography of toucans and barbets, working on a desk beneath the skeleton of a moa, an extinct emu-like bird that stood 12 feet tall and weighed 500 pounds.