sounds like they could be getting a common cold, but I am not 100% sure. Try this links, it was helpful to me.
http://edis.ifas.ufl.edu/ps044 Read it all!
also here is a solution mix to try. And yes, it is perfectly safe, it came from another BYC member.
The solution is 2 cups of household unscented bleach in a 1 gallon milk jug, top off with water & mark it with a sharpie as "solution"
Then use 1 Tablespoon of this in 1 gallon of water for the birds when they are well, and use 2 Tablespoons of it per gallon of bird's drinking water if sick.
I use alot of milk jugs, I fill them all with water, and then measure in 1 Tablespoon per jug, and cap, and put in each coop.
I also put 100% solution into a spray bottle, and use it for cleaning fonts, feeders, incubators, toilets, kitchen counter, eggs.
Eggs go into 2 cups of 120 degree water and 30 squirts of solution for 30 seconds.
Oh, and do not use the solution water in galvinized, rubber or metal containers, just plastic fonts.
