Landscapes, habitat, and migratory behaviour: what drives the summer movements of a Northern viper?

Abstract

Background: Studies on movement behaviour and habitat use are central to understanding the ecology of migratory animals and play an important role in the conservation and management of these species. However, individuals' spatial ecology can vary substantially, and failing to understand differences within or between populations may be problematic. In British Columbia, Canada, where Western Rattlesnakes reach their northern range limit, individuals undertake seasonal migrations between communal hibernacula and summer hunting grounds. Western Rattlesnakes commonly are associated with low-elevation grasslands and open Ponderosa pine habitats; however, recent work has shown that some animals undertake longer-distance migrations into higher-elevation Douglas-fir forests.

Methods: 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. We quantify variation in migration distance, timing, altitudinal migration, home range sizes, and destination habitats used across our sample, and we use a linear mixed-modelling approach to assess potential drivers of long-distance migration.

Results: 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 within-habitat variation was high, suggesting a continuum of migratory phenotypes. Migratory distance was best predicted by two top models: terrain and combined effects (including terrain, physiology, and vegetation factors). Even these top-performing models, however, left much of the variation in migratory distance unexplained (r_s = 0.65 and 0.64 respectively based on k-fold cross-validation where k = 10), suggesting other factors not measured here, such as genetics and prey quality, may also be contributing.

Conclusions: Overall, this study provides critical knowledge on the movement ecology of a far-ranging reptile with implications for the conservation and management of the species in the far north where seasonal movements are commonplace. Our results shed light on some drivers of multi-phenotypic migration in a taxonomic group where this phenomenon has largely been unstudied, while contributing more broadly to a growing body of literature on migratory variation in animals.

Keywords: Crotalus oreganus; Habitat selection; Migration; Migratory continuum; Movement ecology; Multi-phenotypic migration.

Fig. 1

Map of study sites (n = 9) in British Columbia (BC), Canada, where telemetry has been conducted on the Western Rattlesnake (Crotalus oreganus). The green-shaded map area indicates the range of Western Rattlesnakes in Canada (entire range extends south into the USA). Map imaging: Esri; BC conservation data centre (2020). Projection: EPSG3857

Fig. 2

Dot plot showing the frequency distribution of migration distance (MD; m) of individual radio-tracked Western Rattlesnakes (Crotalus oreganus; n = 139) in Canada, colour-coded by (a) the individual’s destination habitat and (b) the study site from which the individual originated. Each circle represents one individual

Fig. 3

Proportion of Western Rattlesnakes (Crotalus oreganus) from the nine study sites in Canada that used each category of destination habitat (Black = Forest; Gray = Open)

Fig. 4

Boxplot showing migration distances (m) travelled by male Western Rattlesnakes (Crotalus oreganus; n = 139) for each of nine sites in Canada. The median is represented by the solid line, and mean is represented by the dashed line. Means not sharing any letter are significantly different by the Tukey test, using square-root transformed migratory distance (MD) values, at the α = 0.05 level of significance. Sample size (n) at each site is indicated below boxes. See Table 1 for a guide to site abbreviations

Fig. 5

Boxplot showing migration distances (m) travelled by male Western Rattlesnakes (Crotalus oreganus; n = 139) in Canada by destination habitat: (a) destination habitat by zone (BEC); (b) destination habitat by category. The median is represented by the solid line, and mean is represented by the dashed line. Means not sharing any letter are significantly different by the Tukey test, using square-root transformed migratory distance values, at the α = 0.05 level of significance. Destination Habitat Zones: BG = Bunchgrass; PP = Ponderosa Pine; IDF = Interior Douglas Fir

Fig. 6

Examples of movement paths of Western Rattlesnakes (Crotalus oreganus; n = 139) in Canada across nine unique study areas (panels). Darker blue movement paths indicate longer migration distances (MD; m). Paths assume straight-line movements between consecutive tracking locations. Shading represents elevation, while forest habitat is coloured in green. See Table 1 for a guide to site abbreviations. Map imaging: Natural resources Canada (2015), forest analysis and inventory branch BC (2021). Projection: EPSG32611

Fig. 7

Scaled model estimates for fixed effects in the top linear mixed models (combined effects [black circles] and terrain [grey squares]) assessing migratory distances undertaken by Western Rattlesnakes (Crotalus oreganus; n = 139) in Canada. Lines represent 95% confidence intervals

Fig. 8

Predicted migration distance of Western Rattlesnakes (Crotalus oreganus; n = 139) in Canada as a function of the two significant predictor variables (a- elevation; b - slope) and one marginally significant variable (c - canopy cover), as predicted by the combined effects model