Possible dry fowl pox, is sure looks like it; may have lost your others to the wet version. Absolutely nothing you can really do about it since you cannot keep wild birds away from your run. Dry pox runs its course in 4 to 6 weeks; (think of it as a common cold, but without the mucous membrane involvement). Wet pox can be (and often is) deadly; treat wet pox as you would with a respiratory illness (aggressively and with antibiotic assistance) with a heaping helping of prayers.
If this is indeed fowl pox, You should be able to introduce new birds to the flock a few weeks after all symptoms have cleared from your existing flock. You should be able to get them now, house them across the yard like you did this last set of birds... and they should be ready when your existing flock is healthy.
I will keep you and your feathered family in my prayers.
