Should I stay, should I go, or something in between? The potential for parasite-mediated and age-related differential migration strategies

Seasonal long-distance migratory behaviour of trillions of animals may in part have evolved to reduce parasite infection risk, and the fitness costs that may come with these infections. This may apply to a diversity of vertebrate migration strategies that can sometimes be observed within species and may often be age-dependent. Herein we review some common age-related variations in migration strategy, discussing why in some animal species juveniles preferentially forego or otherwise rearrange their migrations as compared to adults, potentially as an either immediate (proximate) or anticipatory (ultimate) response to infection risk and disease. We notably focus on the phenomenon of "oversummering", where juveniles abstain from migration to the breeding grounds. This strategy is particularly prevalent amongst migratory shorebirds and has thus far received little attention as a strategy to reduce parasite infection rate, while comparative intra-specific research approaches have strong potential to elucidate the drivers of differential behavioural strategies.