How plastic is migratory behavior? Quantifying elevational movement in a partially migratory alpine ungulate, the Sierra Nevada bighorn sheep (Ovis canadensis sierrae)

Abstract

Migratory species face well-documented global declines, but the causes of these declines remain unclear. One obstacle to better understanding these declines is uncertainty surrounding how migratory behavior is maintained. Most migratory populations are partially migratory, displaying both migrant and resident behaviors. Theory only provides two possible explanations for this coexistence of migration and residency: either these behaviors are fixed at the individual level or both behaviors are part of a single conditional strategy in which an individual’s migratory status (adoption of migrant or resident behavior) is plastic. Here we test for plasticity in migratory status and tactics (timing, distance, and duration of migration) in a federally endangered mountain caprid, the Sierra Nevada bighorn sheep (Ovis canadensis sierrae Grinnell, 1912). We used nonlinear modeling to quantitatively describe migratory behavior, analyzing 262 animal-years of GPS location data collected between 2005 and 2016 from 161 females across 14 subpopulations. Migratory tactics and prevalence varied by subpopulation. On average, individuals from partially migratory subpopulations switched migratory status every 4 years. Our results support the hypothesis that partial migration is maintained through a single conditional strategy. Understanding plasticity in migratory behavior will improve monitoring efforts and provide a rigorous basis for evaluating threats, particularly those associated with changing climate.

Habitat predicts local prevalence of migratory behaviour in an alpine ungulate

“…Migratory status in Sierra bighorn thus appeared to represent a bimodal trade‐off in which migrants prioritize access to nutrition at the expense of increased risk of predation, while residents prioritize predator avoidance at the cost of compromised nutritional intake and increased energetic demands (Hebblewhite & Merrill, ). Unlike many migratory ungulates, Sierra bighorn generally migrate into areas of higher predation risk during the season of allopatry (Fryxell & Sinclair, ; Hebblewhite & Merrill, ; Johnson et al, ; Spitz et al, ). This likely explains the differences in our results from other studies that have shown migrants to be more sensitive to predators at fine, rather than at coarse scales (e.g., Hebblewhite & Merrill, ).…”

“…We used beta regression (Ferrari & Cribari‐Neto, ) to test whether migrant habitat predicted published estimates of the proportion of migrants in each of the eight populations used in our RSF model (prediction 3; Spitz et al, ). These estimates represent a larger sample size ( n = 192 individuals over 12 years; Spitz et al, ) than the data included in this analysis.…”

“…We used beta regression (Ferrari & Cribari‐Neto, ) to test whether migrant habitat predicted published estimates of the proportion of migrants in each of the eight populations used in our RSF model (prediction 3; Spitz et al, ). These estimates represent a larger sample size ( n = 192 individuals over 12 years; Spitz et al, ) than the data included in this analysis. Following Liu, Berry, Dawson, and Pearson (), we defined habitat for migrants and residents as any pixel that had a relative probability of use ≥the mean predicted value for used pixels of a given status.…”

Where to spend the winter? The role of intraspecific competition and climate in determining the selection of wintering areas by migratory caribou

“…Finally, competition experienced on a wintering area seemed to influence the winter range use as individuals were more prone to migrate toward another wintering area the following year when population size was high. In partially migratory species, individuals can switch between migrant and resident status to limit density‐dependent effects (Eggeman et al , Spikes et al ). Here, the degree of competition, and the likely deterioration in winter range conditions that would result, could not only lead to a winter range drift (Schmelzer and Otto ) but also to a shift in the destination of fall migration (Ferguson and Messier ).…”

“…Usually viewed as a predictable displacement between a specific breeding area and a specific non-breeding area (Newton 2012), migratory movements can nevertheless show interannual variations and may vary between individuals in terms of direction and distance travelled (Berthold et al 2004, Bauer et al 2010. In ungulates, although migration can be a fixed behaviour (Sawyer et al 2019), migratory plasticity has received increasing support in recent years (Mysterud et al 2011, Eggeman et al 2016, Jacks et al 2018, Spitz et al 2018) and flexibility in migratory behaviour has been observed in individuals switching between different migratory strategies (Morrison and Bolger 2012, Eggeman et al 2016, Spitz et al 2018. Such variability makes the conservation of highly mobile species particularly challenging (Runge et al 2014).…”