I made one of these halos 2 years ago, and my anecdotal experience is that the older (fully grown) sparrows can't figure it out, but the smaller, younger ones get through. Then, weeks later, I see fully grown sparrows at the feeder, ignoring the halo. Did the little ones "fit" and then grew too used to it to be spooked? Did the big ones learn from the little ones? I don't know.<p>Another observation: Because of the number of sparrows, I did pull the whole feeder down and kept it down for months. Then, when winter approached, I put the feeder and the halo back up, and no house sparrows made it through again. Did the adults who were defeating it before forget? Move on? Die? Who knows!