Swainson's thrushes in migratory disposition exhibit reduced immune function
Evidence indicates that the immune system, which protects an organism from parasitic and pathogenic infections, is frequently suppressed when animals are engaged in activities involving strenuous exercise. We tested the hypothesis that birds reduce immune function during the migratory period in preparation for the anticipated heightened energetic demands of long flights. Swainson's thrushes (Catharus ustulatus), captured in fall, were held in an indoor aviary until January, when migratory disposition was induced in half of the birds with an artificially prolonged daylength. Experimental birds became hyperphagic and deposited fat stores, and then displayed nocturnal activity (Zugunruhe) characteristic of the spring migratory period. Cell-mediated immunity was measured by intradermal injection of phytohemagglutinin in the wing patagium of both control and experimental birds. Birds exhibiting migratory restlessness had a reduced cell-mediated immune response compared to control birds. Our results suggest that birds are immunosuppressed during the migratory period. The suppression may be a nonadaptive response due to unrelated physiological processes, or it may be an adaptive response to the physiological demands associated with migration, such as high energetic demands and the negative consequences of a hyperactive immune system.