The immune system of migrating birds
In the first paper, it is shown experimentally that a mimicked bacterial infection (LPS-challenge) forces autumn migrating birds to prolong their stopover duration by on average 1.2 days in long-distance and 2.9 days in short-distance migrants, respectively (100%-126% longer than controls, respectively). During the prolonged stopover, the immune-challenged birds kept a high "bushlevel" activity (which was unexpected) but reduced their local movements, independent of migration strategy. Baseline immune function, but not blood parasite infections prior to the immune challenge, had a prolonging effect on stopover duration, particularly in long-distance migrants.
To the paper: "A mimicked bacterial infection prolongs stopover duration in songbirds - but more pronounced in short- than long-distance migrants" in Journal of Animal Ecology.
The second correlative study explore how immune function and blood parasite infections relate to several key aspects of stopover behaviour (stopover duration, 'bush-level' activity patterns, landscape movements, departure direction and departure time). The authors show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts.
To the paper: "Immune function and blood parasite infections impact stopover ecology in passerine birds." in Oecologia.
The authors conclude that immune challenges, 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. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory.