Chickens as light sensors

Sunset today, the shortest day, was at 4:19 pm. At 4:00-ish I took some scraps out to the chickens, dragged the dogs away from where they were sniffing at the gate to the pen, and went inside the house. The chickens busily pecked at the leftover parrot food. About fifteen minutes later when I opened the back door again, it was dark enough to trigger the motion-sensor light over the door. The chickens were just filing inside their coop to get tucked up for the night.

So, if you want a motion-sensor light at the back door that goes on when it's getting dark out, all you have to do is rig it up so the chicken will trigger it when she goes indoors. I'm sure there are people who know how to do that.