http://www.outdooralabama.com/fishin...er/fish/bream/
http://en.wikipedia.org/wiki/Bream

I added those two links to illustrate a couple points. Likely the words brim or bream came from Europe, many years ago and are a step more general than sunfish which includes all the sunfishes including bluegill. Looking at the Alabama link, we find most of the sunfish that folks tend to lump together and call brim or bream. The trouble with this, for instance, the really successful fly anglers do not fish for Red Eared Sunfish, or Stumpknockers, the same way you'd fish for Bluegills! Two different species, two different habitats quite often and certainly two different sets of favored foods. Same goes for most of the species of Bream listed in that they all have different niches and so it pays to adjust your presentation, types of flies, etc. Bluegills are not Stumpkknockers, White Crappie are not Black and Pumpkinseed are not Warmouths! Variations, although slight at times, make each of these as different in their habits and habitat as Big insect eating Rainbows are to minnow searching lunker Brookies.

Speaking of Bluegills, which is likely the most widespread of our freshwater Bream or Brim, I've witnessed this behavior quite often and if you'd like to see it side by side with the sipping behavior, fish the same sized Black Gnat and a McGinty, both dry. Most often they'll sip the Gnat while savagely striking, or slapping the McGinty. I figure the difference is that Gnats don't sting while Bees do! They are trying to rip that bee apart before consuming it, or at least get it below the surface where they can inspect for stingers, etc. Don't mistake this example as the end all as far as slapping goes since I am sure that Gills have many reasons for this behavior and my bet is most are mentioned above. Good thread, good responses.