The methods used to study the populations of the Bank vole and Long-tailed field mouse on Skomer Island in 1981 are described. The survey was a repeat of one performed in 1960 and the methods used were almost identical.
The distribution of both species of rodent over the island was studied using trap lines and was found to be similar in both surveys.
The vegetation of the island was divided into nine categories and the habitat preferences of both rodent species assessed. These have remained distinct and, as in the previous survey, the voles were found to be most strongly associated with areas of deep bracken with little undercover and the mice with open areas such as cliff tops and rocky outcrops.
A crude survey of the distribution of the nine vegetation categories was undertaken, a vegetation map was prepared, and the area of each category was estimated. Two trapping grids were established in vegetation categories favoured by the voles. The data from these grids were used in conjunction with the vegetation map to estimate the population of voles over the whole island. Insufficient data were available for such an estimate to be made of the size of the population of mice.