Jean Kroll wrote:I'm just wondering about having the ducks trained to go back into a carrier when they are "done" with their slug-eating work. Also: what can you do to keep them from flying away (I'm guessing there is some way to clip their wings). If a person were to have a small flock of ducks for the purpose of eating slugs out of gardens, how many ducks would you recommend? 
In general, ducks don't train very easily, but they will run to food. If you are planning to handle them a lot or herd them into close quarters, you'll need to spend a lot of time with them. Ducks are usually don't want to get too close to people and panic easily. I would consider a mobile pen rather than a carrier - that way you can just house them close to where you want them to work and they will readily run home when you provide some food.
Most domestic varieties can't fly, so you don't need to fuss with wing clipping. Stay away from the smallest breeds (mallards, harlequins) and flying won't be a problem.
We have a bit less than two acres and 20 ducks. We also have six geese, so we could probably accommodate 30 or more ducks without the geese.