feather plucking

My dear Gal's are having their first half molt. We have two ex-bats and Sybil is a prolific feather plucker!! She's in heaven at the moment with all the lose feathers about but it's getting out of hand, i have 4 semi plucked chickens!! I've tried the violet spray but apart from pretty chickens it hasn't help. She does it more in the morning while they're waiting to be let out to free range. Any ideas how to stop please? it's going to be frost bite all round if it snows?!? Thanks for any advise.

Thanks Joyfull!! i brought them a nice new 12ft by 6ft run for Christmas!
they've got their coop to perch on but i haven't got round to putting their perches up again yet, i'll do it tomorrow! i've hung a sprout stem up but they haven't touched it.
 they've got hemp bedding to scratch in and they get loads of treats, grapes, meal worms, pasta, all the others are fine but Sybil seems to like the taste of feathers as she'll pick them up off the floor.
she did seem to stop for a while but the other gal's molting has started her habit again.
 i have thought about separating her but they've only been a flock for about a month as it took quite a while for my older gals to except them and i don't want to do anything to upset the pecking order again, Sybil is at the bottom of the order!