I thought I'd share a quick bit of info for those that are interested. Two species obviously can't have the same codes, and there is no given preference to any species on who should have "priority", so the reason Cedar Waxwing is CEDW and Cerulean Warbler doesn't have CEWA either is that when conflict arises, both species must conform to a secondary get of codes.
Other examples are: Northern Shrike: NSHR, Northern Shoveler NSHO; Black-throated Gray Warbler BTYW, Black-throated Green Warbler BTNW.
Do a google search for either usgs and bird banding codes or institute of bird populations and bird banding codes and should get you all the info you need.

Everyone has their own personal preference for using codes or not using them but since we just came out of cbc season, thought I'd share a tip, I walk alot on the cbcs I do and HATE and I do mean HATE carrying a clipboard if I dont have to, so I carry a small notebook in my pocket to tally and using 4-letter codes is rather helpful to quickly write the species down and quickly move on, makes using a tally much less a duty and able to focus more on birds for the day.

Scott Downes
downess at charter.net
Yakima WA