Crabapple Island, a 4-ha island covered with a mixed forest dominated by Tilio - Carpinetum Traczyk 1962, is situated on Lake Bełdany (53°42'N, 21°35'E) at a distance of about 120 m from the mainland. Materials were collected by using the CMR (catch-mark-release) method during five 7-day censuses conducted each year in 1994-1998. The recruitment of new individuals to the trappable part of the bank vole and yellow-necked mouse populations was compared with respect to the number of new-captured individuals, their age, and reproductive status. The age of new-captured individuals was estimated from growth curves. It was observed that when the populations of the two species were more abundant, the first-captured individuals were older than when population numbers were lower. The proportion of mature individuals (including pregnant females) in the group of all individuals captured for the first time was higher for the yellow-necked mouse than for the bank vole (48 and 15%, respectively). In both species, the number of mature new-captured individuals was negatively correlated with the total number of conspecific females present in the population. In the case of the bank vole (probably also of the yellow-necked mouse), this may be related with territoriality of mature females. As yellow-necked mice were first captured at an older age than bank voles, this may be a source of errors in estimates of numbers of immature individuals and, consequently, in the analysis of the population structure of this species.
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