Last winter was mild enough to allow most bird species to forage naturally, which explains why the proportion of birds visiting feeders was much lower than usual  The mild winter also allowed many species to stay in their summer range all winter without migrating south, thereby limiting the usual attrition accompanying migration
Hence, last year' s mild winter is responsible for this year' s larger-than-usual bird population
Birds sometimes visit feeders even when they are able to find sufficient food for survival by foraging naturally.