Our brahmas aren't that bad. They're just stuffed up to the point they're mouth breathing. The snot is crusted up inside their nostrils. They're eating, and drinking normally.
Hm, I honestly wouldn't like to say for sure. I don't know for sure if it is pollen effecting them.
I know our vet advises us to give our dog antihistamines during the hot summer months with high pollen. I don't know if you can do that for chickens or even what dosage. I wouldn't like to say.
I'd say just make sure they have plenty of water and shade.
Maybe make a thread about it?