Newly emerging plants provide the best forage for herbivores. To exploit this fleeting resource, migrating herbivores align their movements to surf the wave of spring green-up. With new technology to track migrating animals, the Green Wave Hypothesis has steadily gained empirical support across a diversity of migratory taxa. This hypothesis assumes the green wave is controlled by variation in climate, weather, and topography, and its progression dictates the timing, pace, and extent of migrations. However, aggregate grazers that are also capable of engineering grassland ecosystems make some of the world's most impressive migrations, and it is unclear how the green wave determines their movements. Here we show that Yellowstone's bison (Bison bison) do not choreograph their migratory movements to the wave of spring green-up. Instead, bison modify the green wave as they migrate and graze. While most bison surfed during early spring, they eventually slowed and let the green wave pass them by. However, small-scale experiments indicated that feedback from grazing sustained forage quality. Most importantly, a 6-fold decadal shift in bison density revealed that intense grazing caused grasslands to green up faster, more intensely, and for a longer duration. Our finding broadens our understanding of the ways in which animal movements underpin the foraging benefit of migration. The widely accepted Green Wave Hypothesis needs to be revised to include large aggregate grazers that not only move to find forage, but also engineer plant phenology through grazing, thereby shaping their own migratory movements.

The most common framework under which ungulate migration is studied predicts that it is driven by spatio–temporal variation in plant phenology, yet other hypotheses may explain differences within and between species. To disentangle more complex patterns than those based on single species/ single populations, we quantified migration variability using two sympatric ungulate species differing in their foraging strategy, mating system and physiological constraints due to body size. We related observed variation to a set of hypotheses. We used GPS‐collar data from 537 individuals in 10 roe Capreolus capreolus and 12 red deer Cervus elaphus populations spanning environmental gradients across Europe to assess variation in migration propensity, distance and timing. Using time‐to‐event models, we explored how the probability of migration varied in relation to sex, landscape (e.g. topography, forest cover) and temporally‐varying environmental factors (e.g. plant green‐up, snow cover). Migration propensity varied across study areas. Red deer were, on average, three times more migratory than roe deer (56% versus 18%). This relationship was mainly driven by red deer males which were twice as migratory as females (82% versus 38%). The probability of roe deer migration was similar between sexes. Roe deer (both sexes) migrated earliest in spring. While territorial male roe deer migrated last in autumn, male and female red deer migrated around the same time in autumn, likely due to their polygynous mating system. Plant productivity determined the onset of spring migration in both species, but if plant productivity on winter ranges was sufficiently high, roe deer were less likely to leave. In autumn, migration coincided with reduced plant productivity for both species. This relationship was stronger for red deer. Our results confirm that ungulate migration is influenced by plant phenology, but in a novel way, that these effects appear to be modulated by species‐specific traits, especially mating strategies.

The realized effect of multiple carnivores on juvenile ungulate recruitment may depend on the carnivore assemblage as well as compensation from forage and winter weather severity, which may mediate juvenile vulnerability to predation in ungulates. We used a time-to-event approach to test for the effects of risk factors on annual elk (Cervus canadensis) calf survival and to estimate cause-specific mortality rates for 2 elk populations in adjacent study areas in the southern Bitterroot Valley, Montana, USA, during 2011-2014. We captured and radio-tagged 286 elk calves: 226 neonates, and 60 6-month-old calves. Summer survival probability was less variable than winter (P ¼ 0.12) and averaged 0.55 (95% CI ¼ 0.47-0.63), whereas winter survival varied more than summer and significantly across study years (P ¼ 0.003) and averaged 0.73 (95% CI ¼ 0.64-0.81). During summer, elk calf survival increased with biomass of preferred forage biomass, and was slightly lower following winters with high precipitation; exposure to mountain lion (Puma concolor) predation risk was unimportant. In contrast, during winter, we found that exposure to mountain lion predation risk influenced survival, with a weak negative effect of winter precipitation. We found no evidence that forage availability or winter weather severity mediated vulnerability to mountain lion predation risk in summer or winter (e.g., an interaction), indicating that the effect of mountain lion predation was constant regardless of spatial variation in forage or weather. Mountain lions dominated known causes of elk calf mortality in summer and winter, with estimated cause-specific mortality rates of 0.14 (95% CI ¼ 0.09-0.20) and 0.12 (95% CI ¼ 0.07-0.18), respectively. The effect of carnivores on juvenile ungulate recruitment varies across ecological systems depending on relative carnivore densities. Mountain lions may be the most important carnivore for ungulates, especially where grizzly bears (Ursus arctos) and wolves (Canis lupus) are rare or recovering. Finally, managers may need to reduce adult female harvest of elk as carnivores recolonize to balance carnivore and ungulate management objectives, especially in less productive habitats for elk.

Pronghorn (Antilocapra americana) is an endemic North American ungulate susceptible to negative effects of fences, especially given the vast amount of barbed-wire fencing currently on the landscape. Despite multiple nongovernmental organizations, and state and provincial wildlife agencies publishing guidelines for creating wildlife-friendly fencing, there are no published studies that evaluate and compare evidence of the effectiveness of endorsed practices. We analyzed pronghorn crossing success in Alberta, Canada, and Montana, USA, between 2012 and 2016 in response to fence-modification treatments to understand 1) differences between bottom wire height at selected versus available fence sites, 2) the change in crossing rates before and after fence modification treatments, 3) the effect of a suite of fence, environmental, and demographic characteristics on group crossing success, and 4) the time lag until pronghorn became habituated to different fence modifications after initiation of treatments. Use of either smooth wire or clips with a bottom wire height of approximately 46 cm were most effective at allowing passage by pronghorn, while the commonly proposed goat-bar was ineffective and created a negative behavioral response by pronghorn. Though smooth wire and clips were effective at allowing passage, we observed a time lag as pronghorn switched use from their strong fidelity at known-crossing sites to using modified sites. Pronghorn-group crossing success was greatest during summer, for all-male groups, and increased with larger group sizes. We advocate not using goat-bars as modifications to fences, and instead, recommend using smooth wire and clips at a minimum bottom-wire height of 46 cm to allow movement by pronghorn. Our study provides guidance for wildlife-friendly fencing techniques to wildlife managers and private landholders as a means to improve permeability for pronghorn and additionally, can be used as a model to evaluate fence modifications for pronghorn and other target species that may be sensitive to fence interactions. Ó

Migration is expected to benefit individuals through exposure to higher quality forage and reducing predation rates more than non‐migratory conspecifics. Previous studies of partially migratory ungulates (with migrant and resident individuals) have focused on bottom–up factors regulating resident and migrant segments, yet differential predation between strategies could also be a density‐dependent regulatory mechanism. Our study tested for density‐dependence in mortality, as well as mechanisms of ­bottom–up or top–down regulation in the resident and migrant portions of the partially migratory Ya Ha Tinda elk population. We tested for density dependence in adult female and juvenile survival rates, and then discriminated between predator‐ and food‐regulation hypotheses by testing for density‐dependence amongst mortality causes for adult female elk. Notably, the population declined almost 70% from near previously published estimates of carrying capacity over 10 years, providing ideal conditions to test for density dependence. In contrast to predictions, we found only weak support for density dependence in adult survival and juvenile survival. We also found few differences between migrant and resident elk in adult or juvenile survival, though juvenile survival differences were biologically significant. Predation by humans and grizzly bears was density dependent, but similar between migratory strategies. Predation by wolves was the leading known cause of mortality, yet remained constant with declining elk density equally for both migrant and resident elk, indicating wolf predation was density‐independent. Instead of being strongly regulated by food or predation, we found adult female survival was driven by density‐independent predation and climatic factors. The few differences between migratory strategies suggest equivalent fitness payoffs for migrants and residents. This population is being limited by density‐independent predation leading to declines of both migratory strategies. Our results challenge classical predator–prey theory, and call for better integration between predator–prey and migration theory.