Lower number of post-fledglings in urban areas discovered by a nationwide citizen participatory survey in Japan

The population of the Tree Sparrow Passer montanus appears to be declining in Japan. The decline has been observed during just the last few years, so there are few studies investigating the causes. One of the few studies showed that the number of post-fledglings cared for by parent(s) was lower in an urban area than in a rural area, and thus concluded that factor(s) relating to urbanization may have caused the reproductive success of the Tree Sparrow to decrease. However, the study was limited to a small area within Japan. In order to examine whether the same pattern is occurring nation-wide, we conducted a citizens' participatory survey. In the survey, volunteers counted the number of post-fledglings cared for by parent(s) and posted the data on an internet site. Participants accumulated 406 data entries from various regions. The results were consistent with the previous study: the mean number of post-fledglings was 1.41 in commercial areas, 1.81 in residual areas and 2.13 in rural areas. The data did not allow consistent identification of the breeding stage (e.g. egg stage, incubation stage, and post fledgling stage) for each reported result, which limits interpretation to a degree, but we speculate that factors related to urbanization, for example, insufficient supplies of food and nesting habitats, are possible causes of the reduced numbers of young cared for by Tree Sparrows in urban environments in comparison to suburban and rural areas.