Studies were made in an isolated natural population ofClethrionomys glareolus of the trappability of different age groups of individuals, using either live traps or pitfalls, or both kinds of traps simultaneously. Trappability of bank voles in pitfalls is lower than in live traps. The older the group, the higher the trappability of different groups of individuals in live traps, whereas trappability in pitfalls does not exhibit such differentiation. When live traps and pitfalls are used together differentiation of trappability in live traps is the same as when these traps are used on their own. Trappability in live traps increases with the age of individuals, while that in pitfalls decreases. Individuals introduced into the population are characterized, regardless of their age, by trappability similar to that of the group of youngest individuals in the population. As the age group grows older differences in trappability increase among its members. It was found that an individual's weight does not determine its trappability. The results obtained are in agreement with G 1 i w i c z's hypothesis (1970) suggesting that the trappability of different age groups is due to the social organization of the population.
The effect of population density and the quality of the habitat in the animals' place of birth on the distance and direction in which the young disperse was studied. The material used for analysis was obtained during the period 1968-1971 from individual marking of bank voles Clethrionomys glareolus (S c h r e b e r, 1780) on an island 4 ha in extent, during the period of life in the nest. It was found that the distance covered by the animals in dispersing is in inverse proportion to the density of the whole population. In years with high density 46%> of the young individuals remained at their place of birth, whereas in years with low density the corresponding figure was only 18°/ot. Individuals born in a suboptimal habitat show a greater tendency to remain in their place of birth, whereas individuals born in an optimum habitat occupied mainly by adult individuals move away to the suboptimal habitat. The distance traversed by those individuals remaining in an optimal habitat is smaller than that covered by individuals remaining in a suboptimal habitat.
The average results of assessment and estimation of variations in density were compared, using assessment belts 50, 100 and 150 m. in width. Timidity and distribution of the hares in space were also examined. Over-estimation of numbers decreases with an increase in the width of the assessment belt. A belt 50 m wide is too narrow to assess numbers correctly, as results are distinctly over-estimated when this width is used. The occurrence of hares in concentrations, and their starting up in whole groups tend to create errors in assessment. The number of hares outside the assessment belt is thus dependent on the latter's width. The density of hare populations affects errors in assessment. Estimates of the timidity of hares show a tendency to decrease with increasing width of the assessment belt.
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