Bruun wrote many books and was the co-author of The Golden Field Guide to Birds of North America first published in 1966.

He wrote more than fifteen books including two prize-winning children's books—The Human Body and The Brain with his wife Dr. Ruth Bruun.

Bruun's interest in birds evolved into a passion to use the conservation of wildlife as a bridge for helping advance peace.

In the 1970s Bruun became president of the Holy Land Conservation Fund, a non-profit organization based in New York City that was set up to help support wildlife preservation efforts in Israel.

Bruun sought to expand the work in Israel to the fuller mission of advancing conservation in the Middle East.