I am going to give you the same advice that was given to me and thank got it was. Birds do not get colds. Ignore anyone who tells you otherwise. You need to contact your state vet and have a necropsy done on 1-3 of your birds. I submitted 2 birds for testing. They will be killed. In my state the necropsy was free. Anyone can GUESS what type of Upper respiratory infection your birds have, but there are many. The only way to know is to have them tested, Also.....Once you find out which URI they have you have face the horrible decision on culling the flock or not. The reason for this is because again, birds do not get colds, they get disease. And URI is not curable. They will ALWAYS have it. And whenever they get stressed out, will show signs again. They will pass the disease on to their offspring if you hatch their eggs. If you do choose to keep them, Any bird you ever introduce to your flock WILL get sick as well. Also you cannot consume the eggs after any antibiotic is used for at least 21 days. Your birds could also have Micro plasma. Which again, is why you need to have them tested if you really want to know. I tried to treat mine with antibiotics and I could never live in fear wondering if the chicks in my house would be safe. I culled my whole flock of birds and cleaned and disinfecting with bleach and water and then with dehydrated lime white wash.
I am so sorry your birds are sick. I have been there and I was soooo stressed out daily for at least 3-4 weeks. While it was sad and hard for me to to cull them, I knew in the long run, I was making the right choice. I hope you make the choice that's best for you and your flock. I am no pro, but have been there.