Diversity of den sites used by pine martens <i>Martes martes</i>: a response to the scarcity of arboreal cavities?

1 We compiled a sample of 370 pine marten dens to test the hypothesis that a scarcity of sites leads to the use of suboptimal structures. 2 The sample was influenced by detection methods. Radio-tracking revealed cryptic dens but few natal dens; chance encounters revealed dens in buildings and other man-made structures. A total of 82.8% of natal dens were detected by chance encounters. 3 Most dens were associated with trees (44.3%), rocks (27.6%) and buildings (13.8%). The natal den subsample comprised buildings (44.3%), trees (22.8%), other man-made structures (17.1%) and rocks (14.3%). A total of 69.6% of dens were elevated and typically in structures offering limited shelter. Only 9.8% of all dens were in elevated tree cavities. 4 This diversity of dens reflects a scarcity of arboreal cavities. The alternative structures are suboptimal in terms of energetic costs and risks of predation, and this may limit breeding success in some populations.