Phil,
If you can find some (or know a duck hunter who will save some for you this fall), the small, bronze neck feathers found on the lower portion of the neck work very well. However, it is very, very hard to find them.
Besides, the original spey flies were not tied on hooks as small as AJ #5's and 7's, they were almost always tied on dee hooks (granted not the huge long-shank dees that got up to 3"-6" shank lengths) of our modern size range of #2/0-#2 long-shank salmon hooks. Instead of tying very small speys, why not do like the old salmon fly tyers of the mid-late 1800's did and tie the fly as a standard simple strip wing featherwing with a hackle collar instead of tying it with a palmered spey feather? I.e. tie a Lady Caroline on a standard length #6 salmon hook with a short fine flat gold tinsel tip, G.P. red breast feather stands for tail, olive-brown dubbed body with fine or small oval gold tinsel, a slightly oversized dark grey hackle collar, a 2 turn G.P. red breast feather face hackle, and a bronze mallard rolled wing.
Any of the classic bronze mallard winged spey flies can be tied like this on standard shank salmon hooks. Francis Francis talks about flies being tied fthis way for some of the smaller or more calm flowing rivers. He said they have the somber look of the flies used on the River Spey, but they have a hackle collar and are tied on standard salmon hooks, including some rather small one of 1/2" shanks (about our modern #10's). The Lady Caroline tied this way is particularly effective in summer water, and works wonders when tied true low-water style in the skinny water of late summer/early fall.
