Haydel's sells a really good duck calling instructional CD, and RNT (Rich'N'Tone) does also. Both their brands of calls are very good. Haydel's DR 85 (double reed) is a standby for many. I use one every year to good success. It is very reasonably priced, and blows wet. RNT calls are very good, but pricier. These are mallard calls I'm referring to. Phil Robertson makes the best wood duck call I've ever tried. Woodies aren't easy to call to a set-up, but I was turning them in Arkansas last season and getting them to fly back over the spread to give us shots. It's a good 'un.
Keep in mind, tho', that the best duck call ever made is not as important as a LOT of scouting. Being where the ducks want to be, before they get there, trumps all calling. Good calling is just icing on the cake.
SRH
