No abstract

Summary1. We tested the condition-dependent hypothesis of sexual advertisement in house sparrows (Passer domesticus). Male house sparrows have a bib of black feathers which serves as both a badge of social status and as a cue for female choice. We manipulated environmental conditions during the premoult period of juvenile house sparrows kept in outdoor aviaries. Birds were assigned to two treatments differing in the amount of dietary proteins, which are known to aect the expression of immune response in birds. We tested whether birds in the protein-rich group had better immune responses and developed larger bibs than birds reared on a protein-poor diet. We also checked whether immune response was a predictor of survival and parasite resistance. 2. Individuals with higher cellular immune response at capture had greater probability to survive during the 3 months of the experiment, and they had a higher probability to recover from infection with Haemoproteus sp. (a blood parasite). Conversely, birds with high immunoglobulin concentrations at capture had a higher probability of mortality. 3. Birds on the protein-rich diet had a higher cellular immune response compared to birds in the protein-poor treatment. Humoral immune response showed the opposite pattern, being higher for birds in the protein-poor treatment. We did not ®nd any eect of food quality on the development of the badge, assessed as the size of the trait and its colour properties. 4. In conclusion, our results support the view that immune defences are important for survival and parasite resistance in natural populations, and that they might be costly to produce. On the other hand, we did not ®nd support for the conditiondependent hypothesis of sexual advertisement, suggesting that the badge may not be a costly trait to produce. However, badge size could re¯ect other aspects of condition. The kind of pigments involved in colour signals may be the key factor determining the production costs of such traits.

Nestling birds solicit food from their parents by displaying their open brightly coloured gapes. Carotenoids a¡ect gape colour, but also play a central role in immunostimulation. Therefore, we hypothesize that, by di¡erentially allocating resources to nestlings with more brightly coloured gapes, parents favour healthy o¡spring which are able to allocate carotenoids to gape coloration without compromising their immune defence. We demonstrated that, in the barn swallow Hirundo rustica, (i) parents di¡erentially allocate food to nestlings with an experimentally brighter red gape, (ii) nestlings challenged with a novel antigen (sheep red blood cells, SRBCs) have less bright gape colour than their control siblings, (iii) nestlings challenged with SRBCs but also provided with the principal circulating carotenoid (lutein) have more brightly coloured red gapes than their challenged but unsupplemented siblings and (iv) the gape colour of nestlings challenged with SRBCs and provisioned with lutein exceeds that of siblings that were unchallenged. This suggests that parents may favour nestlings with superior health by preferentially feeding o¡spring with the brightest gapes.