[3] From 1988 to 2005, he held a Royal Society Research Professorship in the Department of Zoology, University of Oxford, where he was based at Pembroke College.

On 15 February 2007, the House of Lords Appointments Commission announced that he was to become a non-party political (cross-bench) life peer.

They have introduced new methods to the science of ornithology, including the use of optimality models to predict foraging behaviour, and, more recently, techniques from neurobiology and experimental psychology to assess the mental capacities of birds and to relate these to particular regions of the brain.

"[12] Having led the Randomised Badger Culling Trials, Krebs became one of the UK's leading experts on bovine tuberculosis.

The findings of the trials led him to oppose further badger culling in 2012 and he contributed to a paper on the subject written by centre-right think tank The Bow Group.