On a recent visit to my lunch hour birding patch, when I first arrived, there was a single brownish duck out on the pond mixed in with the coots. No crisp plumage males were with it to clinch the identification quickly and easily. The i.d. threw me off for some time. I admit that I didn't recognize the species by name and had to open up my field guide once I got back to my car. NatGeo6 is the field guide I had with me at the time, and it helped me identify it as a female Ring-billed Duck.
Same lunch hour, but about a half hour later, the Ring-billed Duck had flown, but there was a different brownish duck. You'd think this i.d. would be easy enough for me, but this one threw me for a loop too. The sharp contrast of the head to the body, right at the neck, along with the yellow eye pointed to Redhead, but the heads wasn't as bright as I'm used to seeing and the body color just wasn't right. When I got back to the office, I pulled out my Stokes guide which is the only field guide I have that shows Redheads in eclipse plumage. That white along the wing is actually part of the folded lighter colored flight feathers as you can see the wing partially stretched out. This dude was in a weird stage of molt that led to some confusion on my part, even though I think I have the right identification.
Kenn Kaufman's Field Guide to Advanced Birding shares some key traits to focus on to aid in waterfowl identification:
Habitat and behavior
Head shape and bill shape
Leg and foot color
Pattern details on dull-plumaged birds
If you can train yourself to note and/or draw these attributes, you will be much better prepared to identify these ducks. Maybe after a few more years and experiences like this one that force me to study my field guides, I will have integrated the knowledge into my hard drive and have instant recognition of these darn ducks.