After being put in charge of the natural history collection of Christian Fleischer he became interested in ornithology, and in 1764 he published Ornithologia Borealis, which included the details of many Scandinavian birds, some described for the first time.

[1] Brünnich corresponded with many foreign naturalists including Linnaeus, Peter Simon Pallas and Thomas Pennant.

He then embarked on a long tour of Europe, spending time studying the fish of the Mediterranean Sea and publishing his Ichthyologia Massiliensis on the subject in 1768.

On his return Brünnich took up the post of Lecturer in Natural History and Economy at Copenhagen University.

& Dalziel published Afrobrunnichia, which is a genus of plants in the family Polygonaceae with two species in West Africa.