It is said that sparrow populations are declining. The sparrow is our most familiar bird, welcomed by many people in their daily lives. Should populations continue to decline, the sparrow too could conceivably face a danger of extinction, following on the path of such once common creatures as medakas and cybisters. Furthermore, a population diminishment on the part of some nearby and familiar species hints at some change in our everyday environment as well. For these two reasons, we are compelled to promptly determine the factors behind this decline. Here, to obtain an accurate understanding of the situation at hand (i.e., just what is happening where), we comprehensively surveyed sparrow observational data obtained throughout Japan by entities ranging from private individuals to government agencies. Armed with this information, we next extracted presumably relevant factors. Field studies, including mark-recapture surveys and DNA analyses, not only revealed considerably more about sparrow ecology than previously known, but also provided an opportunity to discern if the factors extracted above are indeed relevant to these population declines. We conclude with a proposal for how to address declines in populations of familiar/nearby organisms.