Fall and Spring Staging in a Refugia-Dependent Migratory Snake

Animals reliant on seasonal refugia face timing challenges because refugia can act as temporal bottlenecks that limit access to vital resources including food, mates, and suitable thermal conditions. Consequently, the timing of entry into and exit from refugia carries fitness consequences, particularly for temperate migratory reptiles that must balance the threat of low winter temperatures against short active seasons. How such species coordinate migration with refugia use during thermally unpredictable fall and spring transitions remains poorly understood. One potential strategy is the use of staging periods, temporary pauses near refugia that decouple migration timing from entry into or exit from refugia, allowing individuals to synchronize seasonal transitions with favorable conditions. Although staging is well documented in other migratory taxa, it remains poorly studied in reptiles. Here, we used Timber Rattlesnakes (Crotalus horridus) to investigate the existence, timing, and function of fall and spring staging in a refugia-dependent migratory reptile. During the 2021-2022 and 2022-2023 overwintering periods, we observed prolonged fall (average in 2021 = 29 days, 2022 = 8 days) and spring (average in 2022 = 22, 2023 = 26 days) staging, during which snakes remained close to refugia, exhibited limited movement, and entered and exited refugia primarily at night. Movements were consistently associated with two thermal thresholds: on average, ingress into and egress from refugia occurred near ~14°C, whereas fall arrival and spring departure occurred near ~19°C, suggesting temperature may act as an important proximate cue. Consequently, snakes spent over 6 months within refugia, extended to nearly 8 months when staging periods were included, leaving only ~4 months for active-season activities. Our results provide evidence that prolonged staging represents distinct but phenologically flexible phases, functioning as adaptive buffers that likely help individuals navigate thermally variable seasonal transitions. Recognizing staging as a functional component of refugia use has important implications for understanding phenological risk and climate sensitivity in migratory, refugia-dependent ectotherms.