As a professional fly tyer, I have studied the action of flies in the water for over 60 years. I have observed them while SCUBA diving as well as in our test tanks that simulate River current. Sparse flies sink quicker than dense flies. Dense flies exhibit more mass than sparse flies. The point is—there’s a time and place for both kinds. I’ve actually seen anglers throw away flies that were too dense. You don’t need to do that. If you want a sparser fly, simply thin it out. With marabou flies it’s easy. Just keep pinching bits off until you get the desired density. With other materials, you can also pinch them away. We carry small scissors in our vests so we
can trim any fly to the desired density. We also carry a permanent marker to change the color, if desired. So don’t throw away a fly that’s not perfect for the situation. Modify it, instead.