Most of the time, runny nose, especially in an entire flock, to the best of my knowledge, means there's an infectious respiratory disease afoot. Because it's a disease and not a cold or something, most respiratory diseases make the birds carriers for the rest of their lives.
Many of us here (not all) including myself, would choose to cull birds displaying these symptoms.
This is because any bird you bring into your flock would likely contract the same disease. If you choose to keep birds with respiratory disease, it is important that you keep a closed flock = no one new in and no one out. Even though they may get over the symptoms, they will likely still be carriers and contagious.
The only exception i'm aware of is fowl pox, which is accompanied by little sores on the face and comb.
Here is a link to a list of some respiratory diseases. You might be able to do some research on these and narrow down what you're dealing with.