Another goal in tracking the swans’ route and altitude was to prevent collisions between birds and airplanes.
In Antarctica, by banding some 50,000 penguins, Dr. Sladen discovered that penguin parents recognize their own offspring among hundreds of other seemingly indistinguishable young chicks.
He also found that in colonies of as many as 30,000 black and white penguins, couples can identify each other by their distinct cries, beak-lifting and ecstatic flipper-flapping.
By studying penguins, which are highly sociable, Dr. Sladen said, researchers could also gain insights into human behavior. For example, he learned that male penguins can initiate and consummate sexual intercourse even in the worst weather, including gales of 60 miles per hour.
“The sexual impulse must be great,” he wrote, “when these acts are performed under such adverse conditions.”
So, he learned, is the impulse to nourish the next generation. Dr. Sladen found that once female Adélie penguins, which like all penguins are flightless, build a nest and lay eggs, they walk across the frozen sea — sometimes as far as 60 miles — seeking food for their newly hatched chicks. Females can fast for 40 days, during which they lose half their adult body weight.
In 1964, his discovery of DDT residue in Antarctic penguins and seals testified to the pesticide’s vast reach.
William Joseph Lambart Sladen was born on Dec. 19, 1920, in Newport, Wales, to Hugh Sladen and the former Catherine Tucker, both career officers in the Salvation Army. His maternal great-grandparents, William and Catherine Booth, founded the Salvation Army.
Dr. Sladen graduated with bachelor of medicine and bachelor of science degrees from London University in 1946 and later earned a medical degree from London University, where his specialty was bacteriology. After his first Antarctic expedition, he earned a doctorate in zoology from Wadham College at the University of Oxford.
His marriage to the former Brenda Macpherson ended in divorce. In addition to his wife, the former Jocelyn Arundel, he is survived by two children from his first marriage, Catherine Adélie (like the penguin) Sladen and Hugh Sladen, and by two grandchildren.
Dr. Sladen first went to Antarctica in 1948, as a medical officer, amateur biologist and photographer with a British expedition led by Vivian Fuchs. On one of his numerous later visits, he said he had spent more than two weeks alone in a tent after a fire destroyed his base camp and killed several colleagues.
On a continent where temperatures could drop to 100 degrees below zero, Dr. Sladen could be seen wearing only a pullover sweater. Mercifully, the sweater customarily covered a maroon shirt, which clashed with his pastel green pants.
In 1956, he received a Rockefeller Fellowship to come to the United States, where he began teaching comparative behavior and ecology at Johns Hopkins University’s School of Public Health (now the Johns Hopkins Bloomberg School of Public Health). He also continued research supported by the National Science Foundation and conducted with the United States Antarctic Research Program (now known as the United States Antarctic Program).
He later founded and directed Environmental Studies on the Piedmont, which restores and conserves avian habitats and researches trumpeter and tundra swans and other native North American birds. The organization is based just west of Washington, near Dr. Sladen’s home in Warrenton.
“Some of my friends have puzzled over my giving up a medical career for studies in conservation and environmental health,” Dr. Sladen wrote in National Geographic in 1975. But he never second-guessed himself.
His response, he said, was, “Wouldn’t they perhaps trade whatever they are doing to witness the spectacle of 300,000 Adélie 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?”