Irritation, itching, red heads

[Cindyb2]

Today I noticed almost all my chickens have irritation on their heads and are scratching almost constantly! There is some stinging nettles in the area they graze and I’m wondering if it’s from that!? What can I do to help the irritation?

 

[Isadora]

I feel external parasites are more likely than the nettles. Have you checked them for mites or lice?