Migratory common blackbirds have lower innate immune function during autumn migration than resident conspecifics

Animals need awell-functioning immune systemto protect themselves against
pathogens. The immune system, however, is costlyand resource trade-offs with
other demands exist. For migratory animals several (not mutually exclusive)
hypotheses exist. First, migrants reduce immune function to be able to allocate
resources to migration. Second, migrants boost immune function to cope with
more and/or novel pathogens encountered during migration. Third, migrants
reallocate resources within the immune system.We tested these hypotheses by
comparing baseline immune function in resident and migratory common
blackbirds (Turdus merula), both caught during the autumn migration season
on the island of Helgoland, Germany. Indices of baseline innate immune function
(microbial killing capacity and haptoglobin-like activity) were lower in
migrants than in residents. There was no difference between the groups in
total immunoglobulins, a measure of baseline acquired immune function.
Our study on a short-distance avian migrant supports the hypothesis that
innate immune function is compromised during migration.