Just as migratory animals must time life history events to match the availability of resources at disparate geographic locations, specialist pathogens constrained to transmission during only a portion of the annual cycle must track availability of their migratory host. Pathogen prevalence in migratory populations may be controlled via migratory escape from the transmission site, or culling of infected individuals migrating long distances. In theory, pathogens associated with partial migrant populations could avoid this challenge to some degree. For vector-transmitted pathogens, transmission is dependent on the phenology of migrants overlapping with that of the vector, which can shift if climate change-induced warmer springs cause mismatch between location-specific phenology and migrant arrival. Pathogens in populations of partial migrants may avoid the potential for mismatch by taking advantage of the sedentary proportion of the population before migrants arrive. We present a mathematical modeling framework which we use to investigate how shifts in vector phenology at a breeding location impact pathogen prevalence in a migratory population that shifts arrival dates versus shows no response. We then consider the same scenario in a population in which an increasing fraction of the population does not migrate.
In our model, parameterized using data from the literature, phenological shifts that caused vectors to be out of sync with migratory hosts limited the transmission window and drastically lowered pathogen prevalence, especially during peak production of susceptible offspring (“migratory host-pathogen mismatch”). When vectors and hosts were not mismatched, decreasing the proportion of individuals that migrate initially decreased pathogen prevalence through overwinter mortality. However, when vector and host populations were mismatched, decreasing the proportion of individuals that migrated increased pathogen prevalence relative to a fully migratory population by amplifying transmission in sedentary individuals, facilitating transmission in the migratory host upon arrival. Reduced migratory propensity as a function of increased resource availability or more favorable seasonal conditions has the potential to amplify pathogen prevalence in partial migrant populations through “loss of migratory escape” or “sedentary amplification”. In real populations, shifting migratory strategies between years could be a mechanism to escape pathogen prevalence while simultaneously reducing the lifetime probability of migration-induced mortality. Our modeling framework can be extended to investigate a range of scenarios in which host and vector timing, demography, and migration distance vary in response to environmental change.