Abstract
Sympatric breeding of Russet Sparrow Passer rutilans and Tree Sparrow P. montanus was observed during 2013 and 2014 at a camping site at Ogawara Lake in Aomori, Japan. In 2013, both species began to build nests almost simultaneously. Two of three Russet Sparrow broods and two of four Tree Sparrow broods successfully fledged young. The distance of the nearest adjacent nests of the two species was about 2m. Of those neighboring nests, the Russet Sparrow succeeded in fledging young, but the Tree Sparrow failed. In 2014, we established 20 nest boxes at the study site. Six Russet Sparrow broods, including a pair that used a natural cavity, were observed that year. Although Tree Sparrows did not nest in 2014, they visited 700m from their nest only to forage in and below cherry trees at the site. A pair of Russet Sparrows was assumed to have bred in a nest box stuffed with moss, which suggested that the box was used previously by one of the Parus species. Hada (1953) advocated a "Rutilans Line" that demarcates preferable habitat for Russet Sparrows in areas where the average annual temperature is below 10 degree Celsius. Such cool climate conditions occur at this study site due to the “Yamase” cold winds of early summer.
