All of my Marens are sick and no body else.

Hi all,
I have around 80 chickens that free range on our farm. They are happy and healthy for the most part. BUT! All of our black copper marans (8 birds in all) have a respirotory infection that also affects the eyes. They rattle when they breath and develop thick yellow plaques in their eye, under the third eyelid. I am thinking a fungal problem.

But, why are only the marans affected and how do I treat this stuff?

Thank
Suzie
River Rock Family Farm
Utah