Hello bill5335.
I have had success with excluding areas of unwanted IR signature by placing a piece of tape horizontally across the bottom portion of the lens. Experimentation would probably be needed.
Another option would be to aim the motion sensor upwards to exclude the portions with the up-close chickens. Again, experimentation with different aim angles may be needed.
You may want to look at Reducing Sensitivity of Motion Sensor with a Mask - 2
The easiest and guaranteed to work is issue each chicken a heat-shielding suit of Styrofoam or have them socially distance themselves to reduce their collective IR signature to an undetectable level.
The old standby method of placing “No Chicken Trespassing” signs written in the chicken language often works but requires liturate chickens. Of course, I’m kidding about the last three methods.