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I feed most of my chickens a quality layer feed, which is sometimes supplemented with mealworms or other small treats. I also feed my roosters and a few hens a higher protein gamebird grower feed to help them grow plenty of nice, shiny feathers and maintain condition.
Caring for chickens throughout the year is pretty simple. In the summer, you just have to make sure that they have some shade to lay in and always have fresh water available. In the winter, the most important ways to keep your flock healthy are to provide plenty of bedding and feed, minimize drafts, and ensure adequate ventilation. The worst threats to chicken health are cold wind, predators, and unsanitary conditions.
I've never heard of using curtains in nest boxes, or if I do, I unfortunately do not remember. However, Two Crows gave you some good advice on the topic. I would be sure to check out the Learning Center, too--it has lots of great information on various aspects of chicken raising.