He then traveled to Peru where he started work at the Javier Prado Museum of Natural History in Lima, an institution affiliated with the National University of San Marcos.

Along with his wife Maria, whom he met at the University of Kiel and later married in Peru, he spent much of his life studying the Peruvian and South American fauna.

According to François Vuilleumier, curator of the Department of Ornithology, American Museum of Natural History in New York City:[2]The number of topics covered in this monumental work (volume 1, pages 1–789; volume 2, pages 790–1,684) is simply astonishing, and includes the concept of adaptation, death of individuals and of species, homology, systematics, ecological specialization, teleology, convergences, social signalization, mimicry, sexuality, mating systems, and many others.

[1] On 24 December 1971, Koepcke's wife Maria was killed in the crash of LANSA Flight 508 in the Peruvian Amazon rainforest.

Their daughter, Juliane, who was on the flight with her mother was the sole survivor of the crash, having fallen from 3,000 m (10,000 ft) still strapped into her seat.