Increase in Tree Sparrows in urban centers of Tokyo and decrease in the suburbs from 1990s to 2010s

Abstract

Tree Sparrows Passer montanus have been declining in Japan since the 1970s. However, national surveys conducted in the 2010s suggest that sparrow populations are increasing in urban centers, which is considered an unsuitable habitat for sparrows. Therefore, based on data from the Tokyo Metropolitan Breeding Bird Distribution Survey and field surveys, we examined changes in Tree Sparrow populations from the 1990s to the 2010s to determine the causes of these changes. The results showed that the number of sparrows in Tokyo has decreased in suburban areas and increased in urban centers. The total area covered by greenery where sparrows forage also decreased in suburban areas and increased in urban centers, suggesting that land use change affected change in sparrow abundance. However, the area of green space in the 2010s, when the number of sparrows increased in urban centers, was still larger in suburban areas, indicating that the change in sparrow population cannot be explained by the area of green space alone. The number of possible nesting sites is unlikely to be the reason, since sparrows use many utility poles as their main nesting sites, both in suburban areas and in urban centers. Predator raptors were less abundant in urban centers, suggesting that the distribution of predators may have influenced the number of sparrows. The number of fledglingｓ was often only one, and reproductive success was lower in urban centers. The high number of utility poles at nesting sites and the lack of predators may have attracted sparrows to urban centers. Although the low reproductive success may have been an ecological trap, the high number of multiple breeding attempts may have compensated for it. More detailed studies of population dynamics are needed to clarify this.