Cut the partridges lengthways with a sharp serrated knife and hit the pieces so the legs lie flat.
Toast the cumin seeds in a dry pan for a few seconds until they release their aroma. Transfer to a pestle and mortar and grind coarsely, then mix into the butter with the lemon zest.
Rub the partridge all over with the butter and season with salt and pepper. Cook under a hot grill being careful not to let them burn, and turn them several times, brushing with more butter. Cook for about 10-15 minutes, until golden brown and the skin is crisp.