I'd try slowly to introduce him back into the group by adding one at a time and observing to his side of the coop though this way you may see who was doing this to begin with watch as you add over the course of the day. If you find it's more than one having them separated your breaking up the pecking order and once they are all together again this one may not be at the bottom. You can also buy a product called Blue Kote TSC carries it in dauber it will make what ever it touches purple so it disguises any wound or injury and keeps the others for going at it again. It also helps heal. This may take several days to a week to move the pecking order around till this little one is getting picked on.So my husband and I looked when he got home a little bit ago I tried to earlier but the little one was so fussy I couldn't get him to hold still long enough for me to look under his wing. Both my husband and i looked, NOT A SINGLE SCRATCH ON HIM! Atleast it wasn't anything serious however I'm now worried about the other ducklings and whether they will continue to pull out growing feathers..... I'm thinkin they may have gotten more than one feather. Do you suggest me keeping them separated still or would he be ok to go with the flock ???