While waiting for a coffee at the kiosk outside the Coast Café, veteran birder and illustrator Keith Hansen heard a chirp and glanced up at the sky. A few seconds later, he heard another and was ready with a response: “That’s a hooded oriole.”

Two days later, Keith was signing copies of his new book outside the gallery he leases from the Bolinas Museum. He heard the same chirp and shot a glance at fellow birder Josiah Clark. “There’s that hooded oriole!” he said.

Keith’s friends and fans were gathered in the courtyard outside his gallery for the release of “Hansen’s Field Guide to the Birds of the Sierra Nevada,” the first book he’s written but the 13th he’s illustrated. It’s a project more than 20 years in the making.

“It’s not a straight bird book,” said Elia Haworth, the Bolinas Museum’s curator. “He writes in his own voice, which is funny and lyrical, and very knowledgeable.”

The field guide, put out by Berkeley-based independent publisher Heyday, covers more than 250 birds from the mountainous backbone of California. Keith knows each of his feathered subjects intimately.

“With a swaggering stride and sashay of their prodigious tails,” he writes of the great-tailed grackle, “they appear noble if not a bit snobby, proudly carrying their prominent bills aloft.” 

Mr. Clark described Keith’s style as “three-dimensional, psychedelic birding.” 

The gallery courtyard is Keith’s kingdom. He maintains nectar-filled feeders that draw dozens of hummingbirds at a time, and his camcorder is trained on the space, where he sometimes arranges birdseed in patterns or words, through a long spotting scope. He keeps the camera running as birds pick away at the seed, then he reverses the video so the birds appear to be arranging the letters themselves.

Since the day he signed the lease on the gallery in 1991, Keith has been recording every bird he’s spotted from the courtyard. Of the roughly 1,000 avian species ever seen in North America, he’s recorded 230—almost a quarter of the continent’s birds. It’s proof of species diversity in West Marin (Keith is quick to point out that the Point Reyes National Seashore alone is home to more types of birds than live in 44 of the 50 states) but also of his sharp observational skills. 

“He sets the bar for how far away you can identify a bird,” Mr. Clark said. “It’s not about rarity, it’s about accuracy.”

Keith says he picked up his attention to detail from his father, a naval aviator who would direct his five sons’ eyes to the sky every time he heard a plane overhead. “Thirty years later, I’ve turned into my father, but identifying feathered birds instead of metal ones,” Keith quipped. 

It was Keith’s brother Rob Hansen who first captivated him with a bird. In 1969, when the boys were growing up in suburban Maryland and Rob was a Boy Scout, he pointed to a cedar waxwing, identifying its airbrushed brown and gold feathers and the bold black “mascara” around its eyes. 

“That bird changed my life,” Keith said. “I couldn’t believe how beautiful it was.” 

Rob, who spent 36 years as a biology professor at College of the Sequoias, described the moment as Keith’s trigger. “Once that happened, his arc began,” he said. 

By the time the family moved to Fresno in 1970, Keith and Rob were immersed in birding. “We immediately became the little darlings of the local Audubon club,” Rob said. But Keith’s passion was less appreciated by his male classmates, who gave him the nickname “Bird” and beat him up.

“I learned quickly not to really boast about it, but when I’d get out of school I’d go off and I’d be in the forest all by myself,” he said. Those adventures into the mountains began his study of Sierra birds that would eventually yield his new field guide. In between were years of illustrating. 

At an easel in the corner of the gallery, set far away from the window, Keith faces the wall as he works. He starts sketching out birds with a graphite pencil, then lays down many thin watercolor washes, finally going in with colored pencils to push the darks and lift the highlights.  He bases his illustrations off hundreds of hours of footage he’s captured with his camcorder. “There’s a certain feeling you get from things that are drawn in the wild, and I love that look,” he said. “But I can cover more ground if I can go out and document 40 different species in a day with my camera.”

While their father gave Keith his focus on tiny details, “our mother was his biggest art teacher,” Rob said. Janice Hansen taught all of her children to draw, and her influence is still clear in the family. Two of Keith’s other brothers are also artists—Doug is a newsroom illustrator at the Fresno Bee and Craig has worked as an artist and designer at the Lawrence Hall of Science.

Keith first got to know West Marin in the ‘70s while visiting Rob, who worked at the Point Reyes Bird Observatory. As a middle schooler, he accompanied his older brother on trips to the Farallon Islands, where he continued to work until after leaving college. 

He started caretaking the house where he now lives in 1986, but until the pandemic, he and his wife, Patricia Briceño, spent much of their time away from home. They lead birding tours throughout Central and South America, from Southern Mexico down to Guyana and Trinidad. 

Though Ms. Briceño is from the Yucatán, she met Keith at his gallery. She was in Bolinas visiting a boyfriend when she wandered into the gallery and struck up a conversation. Now, she and Keith own a Yucatán property near the border with Belize. They’re planning to travel there again in June, but Ms. Briceño said they appreciated the rest afforded by the pandemic, and Keith was able to finish his book earlier than anticipated.

“I think that everyone needed a break—especially the environment,” she said. 

Keith said he’s seen a significant decline in migratory birds in Bolinas in recent years, but it’s still one of the richest birdwatching places in the world. Between the community of fellow artists and the incredible avian diversity, he feels like he belongs on the Bolinas Lagoon.

“It’s got all the nourishment that any artist could ever want as far as the beauty, the temperature, the political mentality,” he said. “It’s the pinpoint spot on the earth for me to be.”