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.