I have been with three WTC masters and they either use none at all or very little (head only at the time I was with them). With this said it would be nice to hear what others are doing. Some caulk the head area only Thru the eyes Thru the mouth Some caulk the wings Before stitching up into the wings flight web into areas at the elbow Some caulk the tail section Cutting a hole for the caulk Thru the vent hole Some caulk the wing pockets Some caulk the breast feathers. As you can see I've tried to show various places of where I've seen caulk applied. For my self I caulk in the head area thru the eye sockets. I also caulk in the flight web areas as well as the tail. I usually don't caulk at all on a standing duck at the wings but then again I leave my humerus bone to maintain proper anatomy (this is the way I was taught so for me I'll stick with it).