[1][2] He has researched the vegetational and environmental history over the past 10–20,000 years in many parts of the world, including Fennoscandia, UK, Minnesota, the Yukon, Siberia, and Tibet.

On retirement, he became Professor Emeritus in the Department of Biological Sciences, University of Bergen[6] and Visiting Professor Emeritus at the Environmental Change Research Centre, University College London[7] Birks held short-term visiting academic positions in Minnesota, Fairbanks, Kingston (Ontario), Toronto, Lund, Umeå, Abisko, Krakow, Utrecht, Bern, Innsbruck, and Oxford.

[8] He developed and applied numerical approaches in Quaternary pollen analysis with Allan D. Gordon[9] and in palaeolimnology, notably in acid-rain research with Cajo ter Braak[10] and climate reconstructions.

[5] His principal mentors have been Harry Godwin, Frank Oldfield, Herb Wright, Ed Cushing, Derek Ratcliffe, and Michael Proctor.

[5] In 2015, The Holocene published a Special Issue in honour of Birks entitled ‘At the frontiers of palaeoecology’ edited by Richard W. Battarbee, Anne E. Bjune, and Kathy J. Willis.