Migration timing depends on balancing the risks of exposure and escape from infection

Allison Shaw, Raine MacArthur-Waltz , Sandra Ann Binning

As anthropogenic changes alter animal migration routes, understanding the consequences of these changes is critical for conservation. One consequence of altered migratory routes is exposure to infection by new parasites (i.e., migratory exposure). Theoretical models aiming to understand migration-infection dynamics typically have focused on infection-related benefits (i.e., migratory escape) to migration and have not included migratory exposure as a cost, which limits our ability to understand how changes to migration might be constrained by infection-related costs. Here, we explore one way animals cope with altered migratory routes: by adjusting the amount of time spent in different environments to minimize overall intensity of parasite infection. We develop a model of one host moving between two environments (each with an environmentally-specific parasite). We find that if the function describing parasite infection accumulation over time is accelerating, the optimal strategy is for hosts to split their time between environments. However, if both parasite accumulation functions are saturating, it is best for hosts to stick to a single environment. This model generates novel predictions about how time spent by migratory species in different environments is shaped by coinfection with multiple parasites.