Flight initiation distance trials
Between the hours of 0900-1400, we selected subjects for FID trials using a regular sampling regime to increase the evenness of sampling across subjects (Altmann 1974). Focal squirrels were only chosen while foraging and not moving (e.g., walking or socializing) to isolate responses to human approach.
Prior to each assay, the walker noted the location, date, time of day, number of conspecifics within 5 m, and vegetation cover. We considered conspecifics to be present when they were within 5 m of the focal individual; this is the distance this species is most sensitive to environmental changes (Leger et al. 1983; Ortiz et al. 2019). We categorized vegetative cover as high when it effectively covered the body of adults foraging quadrupedally and low when it did not (low = 0-10 cm; high >10 cm) (Owings & Coss 1977; Ortiz et al. 2019).
The walker approached the focal subject at a speed of 0.5-m/sec (Runyan & Blumstein 2004). All walkers were trained to ensure consistent speed and posture. Each walker dropped one marker at their departure location and a second marker at the exact moment the squirrel fled. The distance between the focal squirrel and the flight marker was then measured (± 0.1 m). We also noted the distance between the focal squirrel and (i) the walker’s starting location (henceforth ‘starting distance’) and (ii) the subject’s nearest burrow since these influence FIDs in other species (henceforth ‘distance from shelter’; Bonenfant & Kramer 1995; Engelhardt & Weladji 2011).
After the initial flee response, observers continued to watch the focal squirrel to record its behavior. Squirrels typically took one of two options as their post-FID response: (1) fled into shelter and re-emerged over some period of time or (2) fled and stopped to watch at a distance. If a squirrel ran into shelter, we then recorded how long it took to re-emerge from shelter (henceforth ‘shelter emergence time’). If a squirrel did not run into shelter, we recorded the total distance it fled from the walker (henceforth ‘stop look distance’).