I feed pellets and use a 5gal bucket with a trigger for the birds to get food to drop out of the bucket onto a tray below the feeder.
From the moment I fill the bucket till the point where the food is low enough that it no longer falls out when they activate the trigger, I have a layer of fines sitting on the feeder tray. As long as more pellets keep dropping they leave the fines alone on the tray, but as soon as the bucket stops dropping pellets on demand they suck up all the fines. That is how I tell when the buckets needs to be refilled, as long as there is pellet dust on the tray there's still food dropping out. If I see that the pellet dust has disappeared from the tray, then I know its time to refill the bucket with fresh pellets.
Short story long, if I did not have this feeder tray and the food just dropped onto the ground then all that dust that they eat at the end would go to waste. Plus the fact that I can go 2weeks on a bucket fill.