They've got space - the run is rated for 10 hens and we've only got 4, plus they're getting as much free ranging time as we can manage. But this is happening even when they're free ranging, so I don't think they're overcrowded.
Teh interwebz suggests overcrowding, boredom, lack of protein and bloodthirstiness as reasons chickens do this.
They're not overcrowded. They might be bored, although they currently have a hanging apple, and some veg to pick at as well as their usual food and water. Lack of protein - their layers' pellets are meant to provide everything they need, and I've been digging the garden so they've been happily munching up worms. Bloodthirsty chickens - mebbe.
I'll spray them with the special spray tomorrow, and hang up something else for them to peck at and maybe a free CD or something to keep them amused.
Deborah - it's definitely not ticks, I've checked them thoroughly for ticks, mites, lice etc and I've been religious about making sure the house is thoroughly dusted with red mite powder and the nest boxes have plenty of anti-louse powder.