Foraging–vigilance trade-offs in a partially migratory population: comparing migrants and residents on a sympatric range

Prey often suffer foraging costs associated with antipredator behaviours such as vigilance. Migration is one behaviour in ungulates that can lead to trade-offs between forage acquisition and predator avoidance. However, when forage intake is limited by food-handling time rather than by food encounter rate, ungulates can reduce foraging costs by synchronizing vigilance with chewing (multitasking). In this study, we compared patterns of vigilance, frequency of multitasking and total foraging time between migrant and resident individuals in a partially migratory elk, Cervus canadensis, population while they were together on their sympatric winter range. We used these comparisons to determine whether one herd segment had an advantage over the other in terms of forage intake and predator avoidance. Using observations of focal individuals, we found that residents were better than migrants at adjusting vigilance levels to spatial variation in wolf, Canis lupus, predation risk associated with a human-caused predation refuge. Migrant elk were less vigilant than residents where wolf predation risk was highest. Residents probably had an advantage over migrants because they were better at mitigating the foraging costs of vigilance by synchronizing vigilance with chewing. Migrant elk did not compensate for higher foraging costs by altering total activity time. Our study shows how foraging behaviours of free-ranging ungulates might contribute to demographic differences that lead to the loss of migratory behaviours.