Mothers' Movements: Shifts in Calving Area Selection by Partially Migratory Elk

Abstract

Loss of migratory behavior or shifts in migratory ranges are growing concerns to wildlife managers. How ungulates prioritize safety from predators at the expense of high‐quality foraging opportunities during calving may be key to understanding these shifts and long‐term reproductive success. We compared trade‐offs in selection for forage and predation risk by elk (Cervus canadensis) following 3 migratory tactics (western and eastern migration and resident) during 2 time periods in a declining (by almost 70% from 2002–2016), partially migratory elk population adjacent to Banff National Park in Alberta, Canada. We hypothesized that regardless of migratory tactic, maternal elk would show stronger trade‐offs between high‐quality foraging associated with higher predation risk and forage resources of lower‐quality yet lower risk on calving ranges than on ranges used during summer because of vulnerability of their offspring. Additionally, we hypothesized these trade‐offs would occur at high (2002–2006) and low (2013–2016) elk population sizes. We used a machine‐learning algorithm to predict dates of parturition based on global positioning system (GPS) movements of elk equipped with vaginal implants (n = 60) and predictions were within 1.43 ± 0.85 (SE) days of the known date. We applied the model to an additional 58 GPS‐collared elk without vaginal implants. Based on changes in localized movements, we defined calving areas as the 26 days post‐parturition and compared habitat characteristics of calving areas to 10 similar‐sized areas centered on random locations during summer for the same individual in a latent selection framework. Across the 2 time periods, parturition occurred from 8 May–11 July with median parturition dates differing among migratory tactics and residents shifting towards an earlier parturition date in the later period. All elk, regardless of migratory tactic and time period, selected calving areas with greater forage resources than were available on areas used during summer, with no evidence for greater selection of areas that reduced predation risk at the expense of higher‐quality foraging. Calving season selection for areas with abundant forage exposed western migrants to high risk of bear (Ursus spp.) predation, residents to high risk of wolf (Canis lupus) predation, and eastern migrants to low risk of bear and wolf predation. Patterns in exposure to predation risk during calving between migratory tactics were consistent with the recent decline in western migrants and increase in eastern migrants, implying that conditions on calving areas contributed to observed changes in the number of elk following these tactics. © 2021 The Wildlife Society.

Beyond the encounter: Predicting multi‐predator risk to elk (Cervus canadensis) in summer using predator scats

“…Evidence that a scat-based approach can describe broad-scale patterns of predation risk is supported by the correspondence be- (Berg et al, 2021).…”

“…The study area encompassed the summer range of the partially migratory Ya Ha Tinda elk herd along the eastern slopes of the Rocky Mountains in and adjacent to Banff National Park (Figure 1 ). The Ya Ha Tinda elk herd has declined by 70% from ~1400 elk counted in 2002 to ~450 elk in 2016 (Berg et al, 2021 ). Historically, over 90% of the elk population migrated ~50 km west from the Ya Ha Tinda winter range into Banff National Park.…”

“…Historically, over 90% of the elk population migrated ~50 km west from the Ya Ha Tinda winter range into Banff National Park. More recently, the proportion of resident elk (i.e., elk that remain on the winter range all year round) has increased and more elk are now migrating eastward into areas impacted by forestry (Berg et al, 2021 ; Hebblewhite et al, 2018 ). As a result, the migrant to resident ratio has decreased in the last two decades from 3:1 to 1:1 (Berg et al, 2021 ; Hebblewhite et al, 2006 ).…”

“…The telemetry‐based grizzly bear RSFs were derived from nine bears (6 females, 3 males; Nielsen et al, 2002 ) for the seasons of hypophagia (15 April–14 June), early hyperphagia (15 June–7 August), and late hyperphagia (8 August to denning) in 2004. We updated both wolf and bear telemetry‐based RSFs for years 2014–2015 to account for landcover changes (such as new timber harvest and fires, see details in Berg et al, 2021 ). We used Pearson rank correlations at a random set of points ( n = 1000) distributed across the study area after removing areas of high elevations (>2000 m), bare rock, or ice.…”

Using Piecewise Regression to Identify Biological Phenomena in Biotelemetry Datasets

“…Parturition events can be monitored in real time for studies involving the capture of neonates (Figure 2; Obermoller et al, 2019), or events can be identified post-hoc for retrospective analyses of ungulate breeding and fecundity (Bonar et al, 2018;Long et al, 2009). Previous studies have used piecewise regression to identify change points indicative of ungulate parturition based on movement, but implementation of the mcp package can provide additional analytical options (Berg et al, 2021).…”