Density-dependent and geographical variation in bird immune response

Latitudinal gradients in parasitism are common, causing differences in the intensity of parasite-mediated natural selection. Such differences in selection pressures should affect optimal investment in anti-parasite defense, because defense levels should increase in response to increased intensity of parasite-induced selection. Likewise, latitudinal differences in population density may affect immune responses either by selecting for higher levels or defense, or by suppressing resources needed for mounting efficient immune responses. We tested these predictions in a study of T-cell mediated immune response in altricial bird species in subtropical Spain and temperate Denmark. There were highly consistent levels of T-cell mediated response between nestlings and adults in the two areas, with nestlings having stronger responses than adults. In addition, there were highly consistent levels of immune response in nestlings and adults between the two areas, with responses being consistently stronger in Denmark than in Spain, particularly in adults. Population density was much higher in Denmark than in Spain. We found evidence of density-dependent immune response in nestlings and adults, as shown by differences in T-cell response between study areas being positively related to differences in density. Given that the relationship between density and immune response was positive, we can reject the hypothesis that higher population densities suppressed immune response. Therefore, our results support the hypothesis that birds in areas with higher density allocate more resources to immune response, particularly in adults, suggesting that density-dependent effects of parasitism have selected for allocation strategies that minimize the risk of parasitism.