Geolocator Data Processing
After downloading the data from the recovered geolocators, we corrected
the data for clock drift when possible, and twilight times were
estimated using the BAStag package in R (Wotherspoon et al. 2016;
R Core Team 2020). A light threshold of 1.5 with a dark time minimum of
30 minutes was used to calculate initial estimates of twilight. A
biologist audited this process’s results and edited twilights when they
were obviously incorrect based on the surrounding estimates.
Intervention was rare (e.g., no more than three edits were made per
track) to avoid process reproducibility issues. A 0.8 zenith angle
tolerance eliminated latitude estimates for positions around the
equinoxes. These twilight times were used to estimate initial positions
for each day. Outlier positions more than two standard deviations from
the running 7-day average of twilight times were removed.