During the war, he served as a radar technician with the Flying Tigers aircraft unit in Yunnan, China.
[3] Dennis' work was often through a conservation lens, such as his book The Great Cypress Swamps, where he discusses the importance and history of swampland in the United States.
[2] In particular he was interested in, and searched extensively for, the critically endangered ivory-billed woodpecker in Cuba and in old-growth forests of the southeastern United States.
In 1948, working with Davis Crompton, he traveled to the Oriente Province of Cuba and located a subspecies, called the Cuban ivory-billed woodpecker, after it had not been reported there for several years.
[2] Many ornithologists, including James Tanner, generally regarded as the leading authority on ivory-bills, were skeptical of both the sighting and the recorded bird.