Slipper

Hi, Can anyone help my African Grey,Cofi (a Welsh nickname for native of Caernarfon) has over last few months been pulling his feathers out from around his neck.They seem to grow back but no sooner have they done so and he pulls them out again. I do not believe it to be stress related and he seems in fine form.It has not happened beforeand I have no idea of the cause. Regards Slipper.