ihave seen this type of thing, when shooting primarily from 10am to 5 or 6 pm,, in other words, mid day shooting, when heat waves from the ground and plants begin to shimmer, and judging the shadows on your bison shots, you are shooting at miday in may or june,, due the light color of the calf, i also have the same problen when shooting animals on sunny days in the snow, at least mid day shooting when the sun begins to reflect off the snow and,,,,, it can totally ruin a photograph and cause some focusing problems,,, especailly if you are wanting detail from a somewhat distant subject,, thats my opinion and experiecne from that sort of shot,, in addtion, the compression of the lens on the subject and bg compounds the problem,,