People have already come to this conclusion, but I'll reiterate.
It looks like there is no natural light getting into the roosting box.
It probably gets too dark in there for the chickens to navigate the roosts before they are ready to go to bed, add a window, or electric light.
Hello!
I felt compelled to post because I received 15 baby chicks in the mail last week, which were delivered through an unlucky cold spell, and arrived somewhat shocked. When we received them, three had already died in the box, and several more were threatening to follow behind. I found this...