I just learned this trick. My chickens have enough space which is really the first thing you should consider. However, if you notice excessive feather pulling and you're sure their basic needs are met, then adding some entertainment can help distract them. Place some dead leaves in their run and scatter treats throughout, so they can pick/hunt for the treats. It keeps them busy and entertained. This is what worked for me and it didn't cost anything or take up a lot of time.