I would use the modified for Doves, Personally i think the improved cylinder is to big a spread for bird hunting!
Although Big steel shot such as BB which is what i use for geese does pattern pretty well out of an improved cylinder!
I use full for shooting Block shoots for a tight pattern!
I use improved for trap!
If you want a good good waterfowl choke go with a patternmaster or Kicks choke! Make sure you get the right one for your specific gun! not all chokes are univeral and if you try to force it, you will mess up the thread! Dont do that!
it goes like this
Improved (BIG SPREAD)
Modified (Pretty open) Best overall in my opinion!
Full (Tight pattern)