Interspecific differences in population variability (fluctuations or cycles) can be due to environmental variability, different trophic interactions or differences in movement behaviour under severe circumstances. To establish the most important factor, variations in numbers of small mammals and passerine birds were examined during 11 years along transect lines extending from mature coniferous forests out into cleared areas, thus providing the potential for recording temporal variation at least in suboptimal habitats. Coefficients of variation did not correlate with sample means and were considerably higher in mammals than in birds. Spatial variation was usually larger than temporal variation, except for two small mammal species. Tropical migrants among the passerines demonstrated especially low temporal variation. Correlations with extrinsic variables (variation in weather and food supply) were only observed in resident mammals and birds. The differences in variability between the two vertebrate classes could not be related to food habits or demography but probably to differences in mobility. The later observation calls for consideration and definition of species-specific scales when examining relations between population fluctuations and extrinsic or community factors.