Season the birds with salt and pepper. Stuff each one with sage leaves and a prune. Skewer on the spit and place in front of hot part of coals with a drip pan. Cook until just cooked through (depending on your fire, about 45 minutes to 11/2 hours).
Meanwhile, place remaining sage and prunes into a 1 quart sauce pan with stock, vin santo and tomato conserve. Simmer 30 minutes. Remove birds from spit, cut in half and serve with prunes and roasted potatoes.