Step 1Toast the mustard seeds in a pot over medium-low heat just until they start to pop, 2 to 3 minutes. Add the cider and water to the pot, and stir in the salt, one-half cup maple syrup and bourbon. Add the crushed rosemary sprigs and bring the mixture to a boil. Remove from heat and set aside until the brine cools to room temperature.
Step 2Place the ducks in a large non-reactive bowl and pour over the brine. Place a plate over the ducks to weigh them down so they stay submerged in the brine. Cover and refrigerate overnight.
Step 3The next morning, remove the ducks from the brine and dry them with paper towels. Place the ducks, uncovered, on a rack and refrigerate until about an hour before cooking.
Step 4About an hour before cooking, prepare the smoker or grill to cook over low, indirect heat: Set up a drip pan underneath where the ducks will smoke, and fill with water (or the liquid used to soak the wood chips). Shortly before cooking, adjust the heat as needed to maintain a temperature between 250 and 300 degrees and add the soaked chips to start smoking. Melt the butter in a saucepan and stir in the remaining maple syrup; this will be used to baste the ducks as they cook.
Step 55. Baste the ducks and place them (breast-side up) over the drip pan in the prepared smoker. Adjust the heat as needed (add several coals to either side of the grill as needed if using a kettle grill) to keep the smoker between 250 and 300 degrees; replenish the chips as needed to keep smoking for the first hour. Baste the ducks every 30 minutes or so to keep them moist.
Step 6Cook to an internal temperature of 135 degrees, 2 to 3 hours (timing will vary depending on the size of the ducks and heat of the smoker). To crisp the skin, open the vents of the grill or smoker to increase the heat, and continue to cook the ducks for 5 to 10 minutes more.
Step 7Remove and set aside 15 to 20 minutes to rest before carving.