Migration is a behavior that is currently at risk. Climate change-induced phenological mismatch threatens to de-couple migrants from their resources, and evidence as to whether and how migratory herbivores may cope with changing climatic conditions remains an important question. Variation in behavior across contexts and among individuals can provide crucial insight that can be otherwise obscured at the population-level. Behavioral plasticity quantifies the ability of individuals to modify behavior across ecological contexts and provides an individual-level metric of resilience to future change. Repeatability provides a measure of phenotypic differentiation within a population that suggests the presence of phenotypes adapted for a range of environmental conditions that may provide a buffer against climate change. Quantifying individual-level plasticity and population-level repeatability in migratory behavior to current multi-annual variation in climate will provide insight into how migratory species may cope in a changing climate. We quantified plasticity and repeatability for two migratory behaviors—timing of arrival on summer range and selection for areas with high-quality emergent vegetation along migratory routes—as a function of the timing of spring green-up at the population- and individual-level. We tested our framework using migratory populations of mule deer (Odocoileus hemionus) and elk (Cervus canadensis) in Wyoming (USA).
Results/Conclusions
We found high repeatability in timing of arrival on summer range, indicative of significant differences among individuals (r ± SE, deer = 0.77 ± 0.06, elk = 0.76 ± 0.09). Both populations were plastic in timing of arrival on summer range as a function of the timing of spring (β ± SE, deer = 0.75 ± 0.12, elk = 1.08 ± 0.35), but plasticity did not vary at the individual-level. Repeatability for selection of high-quality vegetation was low for both deer (0.11 ± 0.08) and elk (0.01 ± 0.09). Timing of spring did not affect selection of vegetation for deer (0.02 ± 0.02), but did have an effect in elk, with selection of vegetation declining in years with later springs (–0.13 ± 0.05). Slopes for selection of vegetation varied by individual, suggesting variation in the optimal timing of spring across phenotypes. Our results suggest population-level plasticity to spring conditions in the timing of migration that is homogeneous across individuals, but that consistent differences in the timing of migration results in a distribution of phenotypes adapted to different spring conditions. This cautiously suggests that migratory ungulate populations may have the capacity to be both resilient and adaptable to climate change.