Thanks to Cornell U's Lab of Ornithology, we might have a better understanding of when House Sparrows (HOSP) fully mature. Juveniles are generally immune to halos and other lines/wires devices. According to this infographic, HOSP may not reach adulthood until sometime in mid-Nov, on average, throughout its range. We continue to wonder if continued adaptation is the case, especially if HOSP are resident and mature on site, where you live.
A single HOSP pair can spawn 10 to 20 fledglings per year. If you are one of only a few feeders in the neighborhood, they may be abundant -- regardless of seed type you are using. We know that the Magic Halo isn't always 100% effective, and in some cases, may only "help". Juvenile populations may account for that, since they begin to resemble adults as the post-nesting season goes on. Despite this issue, most folks will acknowledge that they are still better off using a Halo than not. Read about HOSP proliferation, courtesy of Sialis, HERE.