Materials & Methods
The fieldwork for this study was undertaken on the Isle of May (56.1833° N, 2.5667° W), in the Firth of Forth, Scotland. The island is a designated National Nature Reserve of around 45 hectares and consists of a mosaic of coastal grassland with patches of tall ruderal vegetation and frequent exposed bedrock. A feral population of house mice has been present on the island for over a century, and was supplemented by an introduction of 77 animals from the Scottish island Eday in 1982 (Berry, Triggs, King, Nash, & Noble, 1991; Muir, 1885; Triggs, 1991). The population is assumed closed, with no migration. Due to the seasonality of predators (primarily raptors) on the island, including short-eared owls Asio flammeus during the winter, there is assumed to be very little predation pressure.
Eight trapping sessions (see Supplementary Materials) were carried out using a mixture of Ugglan and Longworth live-traps. Trapping effort consisted of transects of twenty traps set in different areas across the island. Trapping locations were selected in order to maximise the coverage of the island within the limits of the terrain, and allowing clearance for bird breeding sites (for a map of trapping locations see Taylor et al, 2019 (Taylor et al., 2019)). Traps were baited with mixed birdseed, provided with hay as bedding material, and checked frequently throughout the day.