Bewilderment: That was the reaction of William J.L. Sladen’s friends when he decided, more than six decades ago, to forgo his medical career and embark on a new one as a zoologist.

The British-born physician would become an internationally known authority on birds, his exploits dramatized in the 1996 Hollywood film “Fly Away Home” and chronicled in the pages of National Geographic.

“Wouldn’t they perhaps trade whatever they are doing,” Sladen wrote in the magazine of his head-scratching acquaintances, “to witness the spectacle of 300,000 Adelie penguins in Antarctica, to round up thousands of pink-footed geese in Iceland, to sit among harems of fur seals on the Pribilof Islands in the Bering Sea, or to take inspiration from the wandering albatross as it soars majestically above the southern oceans?”

All those experiences “have been mine,” he continued, “and they are all unforgettable, yet for me the liftoff of whistling swans from the Chesapeake Bay on spring migration equals or even surpasses, in emotional and scientific impact, those other more exotic adventures.”

Sladen – who was 96 when he died on May 29 at his home in Warrenton, Virginia – was part scientist, part conservationist, part adventurer. He first went to Antarctica in the 1940s as a British medical officer and returned many times for his zoological research, practically commuting to and from the continent in the 1960s.

Once, after a fire destroyed his base hut and killed his fellow travelers, he spent 17 days alone with no shelter but a tent, according to his family. For periods, he subsisted on the meat of penguins and cormorants, another aquatic bird.

“He is a popular figure here,” The New York Times reported in a dispatch from the McMurdo Station research center in Antarctica in 1968, “often seen walking about coatless, wearing only a pullover sweater and a maroon woolen shirt that clashes violently with his pastel-green corduroy trousers.”

Sladen’s research helped reveal the intrigue of Antarctic wildlife including the Adelie penguin, which is smaller – standing roughly 2 feet tall – than the better known Emperor penguin. Adelies, he showed, were capable of recognizing their own chicks among thousands at feeding time.

In a 1966 article published in the journal Nature, Sladen reported that he had detected trace amounts of DDT in Adelie penguins and a crabeater seal, helping to reveal the extent of the environmental threats that the pesticide posed. DDT was banned in the United States in 1972.

Two mountains in Antarctica were named for Sladen, whose research also extended to the North Pole region. He was credited with using radio technology to document the trek of the tundra swan, which covered thousands of miles, from the Arctic to the Mid-Atlantic.

He was perhaps most popularly known for his effort, undertaken with Canadian artist and pilot William Lishman, to teach Canada geese a migratory route from Ontario to the Airlie conference center in Warrenton using an ultra-light aircraft as their guide.

William Joseph Lambart Sladen was born in Newport, Wales, on Dec. 19, 1920. Among his forefathers, his wife said, was William Booth, founder of the Salvation Army. Both of Sladen’s parents served as officers in the organization. He roamed the countryside as a boy, developing an early love of wildlife.

He received two medical degrees in London, including one with a specialty in bacteriology, before completing a doctorate in zoology at the University of Oxford in 1955. Soon after, he settled in the United States, becoming a U.S. citizen in 1962. For many years he was a professor at Johns Hopkins University.