Federal has the right lead free ammo for you. One example is this Ultra-Shok Heavyweight Waterfowl, designed to take down those big birds. This variation, in 12 gauge, has a 3-1/2" shell loaded with 1-3/8 Oz of #6 lead free shot which it fires at 1450 fps.
With a US Fish and Wildlife 2006 blanket approval of several Tungsten based shot types, the door has opened for Heavyweight to wreak havoc on waterfowl. Heavyweight is simply the densest available shot type in the industry. At 15 g/cc, it is better than its closest competitor by 2 on the density scale. That kind of density allows downsizing of shot, more pellets on target and extreme energy. Ultra-Shok Heavyweight will not use the FliteControl Wad like its turkey hunting counterpart, to allow for optimal patterns on waterfowl.