I have read multiple multiple times that NO chickens can not and do not get simple colds. they can however get ALLERGIES - but its usually only presented as runny nose, blocked sinuses, and sneezes. - no wheezing, no foul smelling discharge/crop/breath, will only affect 1 or 2 birds not many/majority. Alleriges are easy to spot, they tend to pop up when you change the coop, or switch bedding material on them...spring/summer arrives with the pollen and such... and again - only 1 chicken in a small flock maybe 2-3 in a real large flock.
I have a d'uccle with allergies (seasonal - lol my daughter's allergies and the chicken work like clockwork)
antibiotics will clear up a respitory infection "cold like symptoms" but will not get rid of the actual bacteria/virus. i will always be present in the chicken.