There are many apps out there for identifying waterfowl through pictures mostly consisting of full grown drakes.
The Waterfowl Identification: The LeMasterMethod app is the mobile version of the popular book used for years by waterfowl hunters. Adapted from the book Waterfowl Identification: The LeMaster Method by Richard LeMaster. Published in 1986 by Stackpole Books.
The app covers 27 North American species, and allows you ID birds by comparing bills to full size illustrations. You can search by bill or species name.
For this review I'll show you the search results for the Ring-Necked duck whose name is deceiving because the ring around its neck is subtle and its uniquely colored bill more distinctive.
All images in the app can be expanded for a larger view simply by tapping on them.
The app was created for iPhone but can be used on iPad and is currently listed on the App Store for $2.99
Screenshots courtesy of app.