Good advice although personally, I disagree with beak trimming unless it is for an overgrown beak. Trimming beaks when birds are young is still done in many commercial hatcheries where hens are going to be kept in cages but I believe Compassion in World Farming are trying to get this banned.
There are a number of possibilities of what could be causing the feather loss - a full or partial moult is my first guess but it could be lack of protein (diet), boredom in a small run, overcrowding, heat stress if around the tail area, pushing their heads through a fence to get grass or something as simple as that.
It can be just one bird pecking the others (usually the one with all her feathers) - if so, she should be removed until all of the others have regrown their feathers.
There are anti-peck sprays - they seem to work (whilst applied and fresh) and if there is one culprit, I have seen people use what they call 'bumper bits' on the beak - these allow the bird to eat and drink but they can't close their beak fully to pull out feathers.