1. Effects of stressors on immune functions have long been studied, but most have focused on chronic stressors, which tend to be immunosuppressive. More recently, an emphasis has been placed on identifying and understanding effects of transient, unpredictable stressors, especially whether and how they affect movement of immune cells to body areas in need of rapid protection. These latter effects have been termed, immunoredistribution.
2. In this study, we surgically implanted small gelatin sponges subcutaneously to measure leucocyte infiltration to skin in house sparrows, Passer domesticus. First, we evaluated whether wild birds exhibit leucocyte infiltration to skin in response to a transient stressor (restraint) as seen in laboratory rodents. Secondly, we examined effects of captivity on leucocyte infiltration of wild sparrows, expecting long-duration captivity to impose chronic stress and thus reduce leucocyte infiltrations. In particular, we expected continuous elevation of the avian stress hormone, corticosterone (CORT), to dampen skin infiltration of leucocytes in birds kept in captivity for short-term (1–2 days) and more so long-term (1 month) periods compared with wild-caught individuals.
3. As in lab rodents, house sparrows exhibited cutaneous leucocyte infiltration, but 1 h of restraint prior to surgery did not influence the magnitude of cellular infiltration. The leucocyte infiltration response was influenced by captivity, however, with short-term captive birds (1–2 days) exhibiting less leucocyte influx than long-term (30 days) and wild individuals. Interestingly, the profile of cellular infiltrates changed with time in captivity with a dramatic shift away from lymphocytes and towards heterophils in long-term captive birds. Placement in captivity did not affect total circulating leucocytes, however, lymphocytes decreased in numbers in wild birds in short-term captivity. CORT followed the expected pattern with the highest baseline levels in short-term captives, intermediate levels in long-term captives and lowest levels in wild-caught birds. Stress responses were not affected by time in captivity.
4. These results indicate that an integral aspect of immunoredistribution occurs in wild birds but future efforts to understand the ecological and evolutionary forces that have shaped this immune response must account for the confounding effects of captivity and attempt to understand the proximate and ultimate basis for heterophilia and lymphopenia with chronic stress.