CYCLE-CORRELATED CHANGES IN THE ACTIVITY BEHAVIOR OF FIELD VOLES, MICROTUS-AGRESTIS

The above-ground locomotory activity of free-ranging field voles in North Sweden was studied with passage counters during different population cycle phases between 1981 and 1988. Characteristic diel activity patterns were associated with different cycle phases. During density peaks, diurnality prevailed, distinct activity maxima occurred at sunrise and sunset and a short-term rhythm of activity was discernible. In late population declines, a peculiar phase-shift from diurnality to nocturnality occurred in July/August. During density low and early increase phases, diurnality prevailed again, but daily activity patterns were not as regular as during the peak phase. The most obvious correlation between population cycle and behaviour, the activity phase-shift during late decline,was recorded in 3 plots differing in vegetation structure, and in the comparison with 3 methods concerning different aspects of behaviour. In contrast, no evidence for seasonal activity phase-shifts was found. The temporal course of an index describing the prevalence of diurnal or nocturnal activity was compared with the predictions from a model, based on long-term behavioural changes. The findings are discussed with respect to changes in vole social structure during population cycles.