Two alternative hypotheses explaining low densities of juvenile wood m
ice Apodemus sylvaticus (Linnaeus, 1758) early in the breeding season
were compared: the Adult Aggression Hypothesis and the Habitat Saturat
ion Hypothesis. They predict different ratios between adult and juveni
le densities, which were tested using trapping data from mixed deciduo
us woodland and from lowland arable field margins. According to the Ad
ult Aggression Hypothesis, juveniles have a poor persistence rate earl
y in the breeding season as a result of aggressive behaviour shown by
the adults. As the breeding season progresses, a drop in adult aggress
ion levels results in increased juvenile persistence, which, in turn,
leads to increased densities. The Habitat Saturation Hypothesis propos
es that juveniles disperse from their parents' territories until the s
urrounding habitats are saturated and that this gradual saturation res
ults in increased densities as the breeding season progresses. The obs
erved correlations between adult and juvenile densities both in woodla
nd and on field margins were consistent with the predictions of the Ha
bitat Saturation Hypothesis.