Fjeldså has authored several books and published numerous research papers, primarily focusing on grebes, birds of the Andes and Eastern Arc Mountains, and evolution of passerines.
[1] Growing up after World War II, he and his playmates also played with leftover ammunition and weapons found in the region.
[1] With his photographic memory and abilities with a binocular and as an illustrator (photography at Russia's border was strictly illegal), he was able to bring new information and details to Norway on an improved version of the Russian AK-47 Kalashnikov rifle, and made detailed plans of the exterior of Russian border stations.
He then joined the Copenhagen Zoological Museum in Denmark as a curator, inspired by Finn Salomonsen and its international research.
[2][3] He has described several bird species and subspecies from South America and Africa, especially the Andes and Eastern Arc Mountains, such as Cranioleuca henricae (Bolivian spinetail) from Bolivia, Laterallus jamaicensis tuerosi (Junin crake) and Atlapetes melanopsis (black-spectacled brushfinch) from Peru, and Batis crypta (dark batis), Sheppardia aurantiithorax (Rubeho akalat) and Xenoperdix udzungwensis (Udzungwa forest partridge) from Tanzania.