No abstract

Many parasites depress the expression of the carotenoid-based colour displays of their hosts, and it has been hypothesized that animals face a trade-off in carotenoid allocation between immune functions and 'degree of ornamentation'. While numerous correlative studies suggest that parasite infection decreases the intensity of carotenoid-based colour displays, the existence of this trade-off has never been demonstrated experimentally in a host-parasite model. In this study, we used the blackbird (Turdus merula) and Isospora (an intestinal parasite) to assess whether this trade-off does indeed exist. Blackbirds were supplemented with carotenoids while simultaneously being exposed to parasites. Supplemented males circulated more carotenoids in the blood and developed more brightly coloured bills than unsupplemented males. In addition, supplementation slowed down the replication rate of parasites. Supplementation with carotenoids enabled infected birds to maintain their bill coloration, whereas birds that were infected but not supplemented showed reduced bill coloration. At the same time, infection slowed carotenoid assimilation in the blood. Overall, we demonstrated that bill colour reflects a bird's health, and that only males with a carotenoid-rich diet are capable of coping with costs associated with parasitic infection. Carotenoids are thus traded off between host physiological response to parasites and secondary sexual traits. Further investigations are required to determine the physiological mechanisms that govern this trade-off.

Lower immune response usually translates into lower fitness. Environmental quality can play a key role in shaping immune responses in the wild, as it influences both resource availability and costly maintenance functions. The aim of this study is to evaluate the effect of habitat quality on proinflammatory response to phytohemagglutinin (PHA) in Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) under contrasting agricultural practices. Specifically, we tested the hypothesis that lower quality habitat (intensive agricultural practices and low temperature) negatively impairs immune responses. To do so, we compared the response to PHA of adult female and nestling Tree Swallows nesting in intensive vs. nonintensive agricultural landscapes over 3 years (2008–2010). In accordance with our predictions, we found that habitat quality affected adult female responses to PHA. The response of adult females was significantly higher in nonintensive agricultural areas than in intensive ones. The level of agricultural intensity, however, had no influence on the response to PHA of nestlings. Our results suggest that female adults can buffer the negative impact of a low-quality habitat on their nestlings. Furthermore, our results suggest that under limiting conditions, individuals may have to trade between immunity and other functions such as parental effort.