The single most important aspect of this outdoor technique is the fire; good coals, and plenty of them, capable of keeping a 375°F to 425°F heat for 30 minutes. How much fuel that takes depends on wind, temperature, elevation and your pit, but at sea level I want 8 fireplace logs the size of my arm or 10 pounds of charcoal, either one reduced to glowing coals. The whole ducks should be rinsed and drained, dry and room temperature. Rub each duck inside and out with the dried juniper berries, crushing them against the skin; throw the hulls into the cavity. Sprinkle salt, fresh pepper and rosemary over the whole birds, and stuff them with onion. Plug the birds with a stalk of celery or an apple quarter. Put the grill near the coals and place the birds breast down until singed and slightly colored, then turn them back side down until juices from the breast run clear when pierced.