Hello,
When looking at this problem as a whole you might be having an issue with insects or a small parasite. If it was limited to only one bird getting pecked than I would believe the cause is limited to him/her. However, since this is going on with your entire flock I would suggest the following...
Since you will need to gather some additional information do the following...
1.) Inspect your birds very closely. Make sure no visible parasites or insects are near the pecked areas.
2.) Check the area around the location where the pecking takes place (e.g., backyard, coop, chicken run, etc.).
3.) Isolate any chicken who seems to be the aggressor. He or she might be starting the problem...and the others live by example.
If you don't find any parasites or bug issues than I would work on isolation. If you do find something or the problem does not resolve itself than repost your question hear for more assistance...