The animals in Fitzgibbon's field seem rather intelligent. Sure, it could just be typical cartoonish anthropomorphism, but perhaps the injection given to the rats and mice of NIMH was contagious somehow. The rats and mice got the concentrated dose, so they're far more intelligent, but Mrs. Brisby (whose mate was a NIMH escapee) and her family seem quite smart, even compared to their 'neighbors'. They have a fully furnished home and wear clothes (The Shrew picked up the habit, or perhaps caught the 'bug' from Mrs. Brisby). The Great Owl is so smart because he ate one of the rats of NIMH, thereby getting the 'full' dose (Hell, he even LOOKS a bit like Nicodemus).