Studies of a Small Mammal Population in Bagley Wood, Berkshire

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Abstract
721 individuals chiefly the wood mouse, Apodemus sylvaticus, and the bank vole, Clethrionomys glareolus, were caught and recaught 2460 times between Oct., 1936, and Feb., 1939, through use of the Tring live trap. Each mammal was marked with a numbered ring made from sheet nickel and placed around a hind leg. Two grids were used for trapping with the traps set 150-200 yds. apart and covering 6.2 and 4 acres of easily separated habitat types. Each monthly census consisted of 4 consecutive nights of trapping. About i of the population disappeared within 1 month after their first capture. Decrease in population to a low density in the spring of 1938 together with an increase in number of deaths inside the traps suggested a mortality factor involved. Apodemus was wider ranging than Clethrionomys, but in both spp. most individuals restricted their activity to a fairly small home range, 100 and 60 yds. or less, respectively. Both immigration and emigration of individuals in the trapping areas caused fluctuations in the population density as also did changes in weather. The sex ratio of the entire catch showed equal [male][male] and [female][female], but [male][male] predominated in the spring and autumn. As the mammals were weighed when caught, there was apparent an increase in wt. with the onset of the breeding season. Certain habitats in the area were unexplainably avoided by Apodemus, but with Clethrionomys this was correlated with the scarcity of bracken fern, Pteris aquilinum. The population survival during a period of depression was relatively higher in poor habitats supporting a low population to begin with, and it is suggested that'' such habitats may be essential for the perpetuation of the species.

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