There are a number of respiratory diseases that chickens get, none of which are good news. They often have similar symptoms,which can vary bird by bird. The only way to tell for sure is to have samples tested by a lab. These include Mycoplasma; Infectious bronchitis; Infectious laryngotracheitis. the most common is Mycoplasma gallisepticum .
Antibiotics won't kill the infecting organisms but will control secondary bacterial pathogens.
Even if a chicken gets better they usually become carriers, and infect new birds, and symptoms can re appear each time the birds are stressed. The key to management is to reduce stress.

It is very difficult to maintain a flock free of Mycoplasma. It can be done, most practically by starting over with a flock from a Mycoplasma free source. but it's tricky to keep them Mycoplasma free over time, if they are free ranged or housed outdoors.

If it's not disease, it could be that your coop has poor ventilation leading to ammonia build up from manure. Is your bedding dusty or wet? Is there manure build up? Poor ventilation and ammonia lead to respiratory symptoms too. If you can smell ammonia it's way too high and will affect your chickens.
If you are feeding just scratch grain, that does not provide enough nutrients as the sole source of feed. It's thought that chickens eat might start to eat eggs to get more protein. You can buy age appropriate feed from any feed mill that supplies chicken products, that has added vitamins and minerals. Feed should be fresh for optimum nutrition. Scratch is OK as a treat but they should not get too much of it.