From what I know of deer behavior, they are quite happy to eat corn right off the field, so much so that deer from agriculturally-worked areas (the flat areas of northwestern Ohio, for example) taste less "gamey" than wood-dwelling deer of hilly areas (in southeast Ohio).  Maybe there is a way to use hunting data and use one as a control group against the other.

One possibility not mentioned is the removal of an environmental contaminant:  1980 corresponds to phase-out of leaded gasoline.  Could it be that lead was making us (and all the animals) both dumber and skinnier?