Guy Tudor, a self-taught ornithologist, wildlife illustrator, and MacArthur fellow whose field guides to birds trained generations of naturalists and citizen scientists about how to distinguish a rufous-breasted leaftosser from a lesser hornero, died July 14 in Greenport, N.Y. He was 91.

His death, at Stony Brook Eastern Long Island Hospital, near his home, was caused by pneumonia, Claudette F. Welch, a friend, said.

Digital avian databases like eBird were far in the future when Mr. Tudor first set about documenting swaths of the avian world in watercolors and gouaches renowned for their delicacy and precision, and for the artist’s singular knack for capturing the quiddities of winged creatures with paint and brush.

“Guy liked birds like the spadebill, with a lot of character,’’ Sophie Webb, a biologist, wildlife illustrator, and author, said.

Despite its classification as a tyrant flycatcher, she added, the diminutive spadebill is so notably shy that it might be a washout in the forest but for its voice, an assertive trill that broadcasts distinctly short-king energy.

It was Mr. Tudor’s ability “to capture the unique behavior, motion and exact plumage of obscure species’’ that earned him a MacArthur Fellowship in 1990, said Terence Clarke, an artist and naturalist who was a friend of Mr. Tudor’s.

The spadebill’s typical range is from southeastern Mexico to Central and South America. So was Mr. Tudor’s.

For much of a career that spanned decades, he pursued the oscines, or songbirds, of the Neotropics to their habitats in the Southern Hemisphere — sketching, making compendious field notes, and amassing a vast file of images that he used as a reference tool when painting the plates for definitive books including “A Guide to the Birds of Colombia’’ (1986), by Steven L. Hilty and William L. Brown. It and many of Mr. Tudor’s other illustrated books, including “A Guide to the Birds of Venezuela’’ (1978) and “Field Guide to the Songbirds of South America’’ (2009), became landmarks in the field.

In both work and life, Mr. Tudor pitched his ambitions as much toward avocational birders as professional ornithologists.

“Guy was a born mentor and teacher,’’ Clarke said, who helped “launch the careers of literally generations of young scientists, field researchers, and wildlife artists, despite being a total curmudgeon.’’

His nearly lifelong base of operations was a modest four-room apartment in Queens, N.Y. There, in collaboration with ornithologist Robert S. Ridgely, he produced, among other books, the multivolume monument that is “The Birds of South America’’ (1989 and 1994). Almost 1,700 species, or roughly half of all identified species found between the Isthmus of Panama and the Tierra del Fuego archipelago, are represented in its pages.

Hunched over a drawing board with an ever-present American Spirit cigarette dangling from his lips, Mr. Tudor conjured with pen and brush the hosts of the tropical heavens — spadebills, yes, but also such resplendent creatures as the keel-billed toucan, the scarlet macaw, the turquoise-browed motmot, the paradise tanager, and the long-wattled umbrellabird.

Guy Adwill Tudor III was born Sept. 28, 1934, in New York City to Charles Tudor and Margaret (Matthews) Tudor.

His father was a journalist, artist, and art director employed by the Resettlement Administration in Washington who worked alongside photographers such as Walker Evans, Carl Mydans, and Dorothea Lange to create a portrait of rural poverty during the depths of the Depression.

He would go on to become a founding art director of Life magazine and is best known for designing the magazine’s indelible logo.

“My father and grandmother were both illustrators,’’ Matthew Tudor, Mr. Tudor’s brother, said in an interview.

“I don’t remember my father doing illustrations of animals, but that’s what my brother started out with, birds and then caterpillars and butterflies — anything with wings.’’

Guy grew up in Manhattan and attended the private boarding school Governor Dummer Academy (now the Governor’s Academy), then Princeton and Yale. Later, he served in the Army.

He was married twice, to Elizabeth Linko and Michelle Le Marchant. Both predeceased him. A son from his first marriage, Michael Seton Tudor, died in infancy. Mr. Tudor’s brother is his sole immediate survivor.

In a life marked by rebelliousness and wanderlust, Mr. Tudor found in birds a dependable constant, his brother said. “He was very fond of noting that he had gone to 50 countries logging his birds.’’

Matthew Tudor added: “Once, he got off a plane in Caracas carrying a telescope he had affixed to the butt of a rifle and immediately started pointing it around on the tarmac, looking for birds. Half the military of Venezuela came charging at him.’’

It was not only birds that fascinated Mr. Tudor. He revered all winged creatures.

In 1984, he helped found the New York City Butterfly Club, serving as its longtime president and hosting meetings in his cramped apartment.

He went on to be one of the founders of the North American Butterfly Association, whose annual census counts have starkly illuminated species decline resulting from threats by pesticides, habitat loss, and climate change.

“Guy was a deeply passionate conservationist,’’ said John A. Gwynne Jr., a bird illustrator and a founder of the storied Sakonnet Garden in Little Compton, R.I., who collaborated on field guides to the birds of Venezuela and Brazil with Mr. Tudor.

“People claim Guy was a curmudgeon, but it’s really that he was alarmed — and rightly so — by what is happening to the natural world,’’ Gwynne added.

“He always used to say, ‘The infidels have invaded my church, and they’ve almost reached the altar.’’’