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’).