I have a walk-in coop that is 6x9 and has sand for the bedding. The roosts are two different heights and neither is too high. For about 2 weeks now, 4 out of my 15 chickens have started staying out in the yard after dark and ending up sleeping on the lit porch of our shed. These same 4 (never any others) will still be scratching and pecking around the yard even as late as 11pm. What could have made just this small group stop going to the coop?