Overwintering in nests on the ground in the harvest mouse

The harvest mouse Micromys minutus has adapted to inhabit the stalk zone of grassland vegetation and is identified by the characteristic nests this species builds on grasses above the ground. Such aerial nests have been considered as an almost exclusive sign of harvest mouse populations and have therefore also been used to determine habitat choice and population density of the harvest mouse. However, we found that nests built on the ground occur after farmers burn away grasslands dominated by native grass Miscanthus sinensis and Pleioblastus chino. The aim of this study was to determine whether the harvest mouse habitually builds this type of nest and, if so, when it occurs. Because it was difficult to locate nests on the ground when the soil was covered with grasses, we located all the nests in the native grasslands after burning in the spring in 2 consecutive years. We then attempted to estimate nesting time by comparing the mineral content of nest materials to mineral dynamics in M. sinensis leaves. Nests were distributed over all the grasslands investigated, and nesting times were calculated to occur between early September and middle March. Our findings demonstrate the harvest mouse repeatedly constructs this type of nest for overwintering. Considerable nondetection errors would have occurred with regard to this species by exclusively interpreting data on the presence or absence of aerial nests.