Abstract
The alien red-bellied tree squirrel, Callosciurus erythraeus (common name : Formosan squirrel), is common in woodlands of Yokohama Nature Sanctuary, Kanagawa Prefecture, Japan. By the route censuses conducted in the early morning, 1.2 individuals were observed per hour regardless of vegetation types. The number of observed squirrels was lower in winter than the other seasons. Especially, the squirrels were not active in the morning less than 0℃ in temperature. From December to March, the number of squirrels acting outside the nest increased with rising temperature in the morning. Therefore, the low temperature in winter may be one possible factor restricting further expansion of this alien species in Japan.
