Fifty burrow systems of Microtus arvalis were excavated, from October 2006 to August 2007 in wildflower fields and quasi-natural habitats in five areas near Bern, Switzerland. They comprise an aboveground part, which is longer in spring/summer, and a subterranean part, which is longer in autumn/winter. Three main construction types exist: (a) linear burrows, (b) compact and tight networks, and (c) structures containing both a compact network around the nest and linear parts elsewhere. The subterranean length was on average 16.9 m (range: 0.5–70.2 m) and aboveground length 39.4 m (range: 0–95 m). Numbers of intersections, dead ends, and openings correlate significantly with subterranean burrow length. Nests and food caches were located at a maximal depth and mostly in a central position, as revealed by graph theory.
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