Immune function as a driver of variation in stopover ecology and behaviour
Migration usually consists of intermittent travel and stopovers. Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood.
Many physiological changes occur in migrants in order to enable a successful migration. One such adaptation is modulation of the immune system. The immune system protects the body from diseases and is important for survival. At the same time, it incurs costs in terms of production, maintenance and activation. This applies in particular to innate immune responses because they include fever, reductions of movements and loss of appetite.
Our recent work has shown that sick birds have longer stopovers and move less during the stopover. Longer stopovers of infected birds are the result of the costs of an immune response per se rather than poor physiological condition of already sick birds. Furthermore, we found evidence that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages.
Incorporating measurements of immune function into migration research adds a new level to the understanding of the ecology and evolution of migration. Accounting for these effects will also enable us to fine-tune and apply optimal migration theory. Finally, quantifying how sickness behaviours influence stopovers and subsequent migration is crucial for our understanding of how diseases spread.
Future research ideally includes following and sampling individuals throughout their migratory journey as well as measuring other physiological systems.