Hi... I think that more info is needed to get good responses. How old are they? And, while they free range -- I assume they have a coop in which to roost at night? Someplace to be out of the weather, sheltered from the wind and rain?
You can use the Search here on BYC for "runny nose" or "nasal discharge" and see what you come up with... if it is a small amount and clear... maybe just keep an eye on it and see if it goes away -- was a one time thing? Watch all of them for other symptoms. Are they laying eggs? Molting? What do you feed them?
Depending on their age... all chickens get worms, and free range chickens are most prone to worms... Have you ever wormed them? Not that a mild runny nose with no other symptoms means worms or anything else... but... consider worming them if you haven't (if they are adult, full grown or over six or seven months old). You can Search here on BYC for "worming with Valbazen".... "worming".. "Valbazen dosages"... "giving oral medications".. "Tapeworms"... "egg withdrawal with Valbazen".. etc.. note info from dawg53... good info from him on worming, dosages, etc.
Good luck.