Geographic Variation in Clutch Size of the Varied Tit in East Asia: Latitudinal Gradient along the Japanese Islands

Geographic variation in the clutch size of Varied Tit Sittiparus varius, a common nest-box breeder in East Asia, was reviewed and analyzed using a parametric bootstrap approach to investigate the relationship between clutch size and latitude, subspecific classification, and habitat size. We collected 49 data sets from study sites ranging from 26°46″N to 42°40″N and found an evident geographic variation in the local mean clutch size ranging from 3.7 to 7.6 eggs. Parametric bootstrap analyses showed a latitudinal increase in clutch size, with an average of 0.184 eggs per degree. Population isolation as a subspecies was identified as another influential factor in clutch size variation for one of the four subspecies examined, i.e., S. v. owstoni, which inhabits small islands far south of the Japanese main islands. However, the effect of habitat size (islands smaller than 100 km2) was not significant, although the clutch size on some small islands tended to be smaller than that of nearby larger habitats. Such a variation in clutch size of the Varied Tit indicates there is a conspicuous geographic variation in its life history. The clear latitudinal gradient of clutch size observed in the Varied Tit may help us to understand environmental influences better, as the factors that change independent of latitude may have a lower contribution to such a clutch size variation.