Western rattlesnake (Crotalus oreganus) migration and habitat use in British Columbia, Canada

Studies on migratory behaviour and habitat use are central to understanding the ecology of migratory animals and provide important data to inform the conservation and management of these species. However, spatial ecology can vary substantially among conspecific individuals, and failing to understand differences within or between populations may be problematic. In British Columbia, Canada, where Western Rattlesnakes (Crotalus oreganus) reach their northern range limit, individuals undertake seasonal migrations between communal hibernacula and summer hunting grounds. Mature snakes also show annual fidelity to their migratory paths, providing a useful system to examine the development of migratory behaviour and variation within and between populations. Knowledge on the spatial ecology and habitat use by juvenile vertebrates in general is sorely lacking, and we address this gap by comparing spring outbound migratory movements of juveniles to those of adult Western Rattlesnakes at a site in southern British Columbia (BC), Canada, and describe habitat use by juvenile snakes along their active-season movement paths. Juvenile movement data were collected in 2021, and adult data (2011-2016) were drawn from a long-term project database. We found that compared with adult rattlesnakes, juveniles displayed similar directional orientation, direction of vertical migration, and path sinuosity, but initiated spring migrations later and exhibited shorter movements in terms of distances and rates. To assess juvenile habitat use, we measured habitat at two spatial scales along the active-season movement paths of snakes. Resembling adult rattlesnake behaviour elsewhere in BC, juveniles selected habitat features that provided structurally stable cover (e.g., woody debris, shrub, and rock cover). Additionally, we tested for differences in features at sites used for short- and long-duration stopovers but identified little difference in the microhabitat features associated with these two categories of stopover locations. Western Rattlesnakes are commonly associated with low-elevation grasslands and open Ponderosa pine (Pinus ponderosa) habitats; however, recent work has shown that some animals undertake longer-distance migrations into higher-elevation Douglas-fir (Pseudotsuga menziesii) forests. Here, to further investigate multi-phenotypic migratory tactics and habitat use, we compiled all available raw data from radio-telemetry studies conducted on adult males (n = 139) between 2005 and 2019 from nine study sites across the Canadian range of Western Rattlesnakes. On average, snakes migrated 1364 ±781 m (ranging from 105 m to 3832 m) from their overwintering dens. Migratory distance differed significantly between sites and was higher among individuals using forests as their migratory destination, yet there was immense variation in migratory distance among snakes using forests and open habitats, suggesting more of a continuum of phenotypes than a dichotomy. Next, we used a linear mixed-modelling approach to assess potential drivers of long-distance migration and found that migratory distance was best predicted by a combination of physiological factors, landscape terrain, and vegetation. Even the top-performing model, however, left much of the variation in migratory distance unexplained (rs = 0.65 based on k-fold cross-validation where k = 10), suggesting other factors not measured here, such as genetics and prey quality, may also be driving long-distance movements. Overall, my thesis provides critical knowledge on the ecology of a young age class of rattlesnake and sheds light on some of the drivers of multi-phenotypic migration among mature snakes. The findings have implications for the conservation and management of rattlesnakes in the far north where seasonal movements are commonplace and contribute to our growing understanding of the complexity of patterns and variation in the movement ecology of this far-ranging snake.