My free range hens do this to me.
Melissa, who recently adopted some ex-battery hens from a poultry farm found the same thing was happening with to her every time she let them out of their shed, an was wondering if this was a problem.
So what is really happening?
Its actually the ultra violet colours that they are pecking at. They do it most especially to my jeans and black clothing. It goes back to what we wash our clothes with, which is phosphorous. Phosphorous is visible in their spectrum.
Their range of vision is vastly different to humans, so they are seeing things that we cannot see in the light spectrum. They are fascinated with light that encourages them to seek out and peck.
In nature, bugs actually give off a light spectrum which tells the hens where they are.
Its a normal behaviour that is built into poultry.
I reckon its rather amazing to watch and I love it when my girls come over to preen me. Yes, they can get a little too pecky at times but they are not trying to hurt.