I just read Speckledhen's Ten Commandments of Good Flock Management ... #8 says birds do not get colds, they carry diseases & are like that for the rest of their life. The way to handle this is to cull them. Seriously? Could this be the issue to my birds?
But if so, what would make the next new batch any different ??
First, stop giving meds. You are trying to crutch up poor immune systems. I'm guessing you started with medicated feeds? Vaccines?
Second, if you have birds that rarely or never show any symptoms of illness, mark these. Keep an eye on them and see if they continue to stay relatively healthy. Sniffles and such are not a major problem. Weak, listless, chronic diarrhea and parasites are signs of a weak immune system...these are the birds to cull.
Keep all the healthy birds, kill the rest. Don't start over....stick in for the long haul. Breeding for health, hardiness and other traits you want takes some time.
Using deep litter in your coop and keeping it turned and dry will help keep your birds healthy. They need to have a certain exposure to a natural environment to develop a good immune system. I throw black oil sunflower seeds in my deep litter and let the birds keep it fluffed for me. I only clean out my coop twice a year...there is never any smell and I never have had a sick bird.
Use a little unpastuerized apple cider vinegar in their water~
not in a metal waterer~ at all times. This is a natural immune booster and can help with digestion of nutrients.
If you are presently disinfecting everything, stop doing so. Your chickens need to live in their environment...their natural, chickeny environment. If they cannot live in this, they can't live anywhere. This is where you live, this is the building, run, nests in which they must reside. They either get tough and live, or they get sick and must be culled. The tough ones are the ones you keep and breed to produce more tough ones.
If you truly want healthy chickens, you must stop supporting and medicating birds with sick immune systems and keep only the birds who developed an adequate one. Then help them do so even more.
Good luck with your flock!