Immunocompetence and condition-dependent sexual advertisement in male house sparrows (Passer domesticus)

1. 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 fe male choice. We manipulated environmental conditions during the premoult pe riod of juvenile house sparrows kept in outdoor aviaries. Birds were assign ed to two treatments differing in the amount of dietary proteins, which are known to affect the expression of immune response in birds. We tested whet her birds in the protein-rich group had better immune responses and develop ed larger bibs than birds reared on a protein-poor diet. We also checked wh ether 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 blo od 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 com pared to birds in the protein-poor treatment. Humoral immune response showe d the opposite pattern, being higher for birds in the protein-poor treatmen t. We did not find any effect of food quality on the development of the bad ge, assessed as the size of the trait and its colour properties, 4. In conclusion, our results support the view that immune defences are imp ortant for survival and parasite resistance in natural populations, and tha t they might be costly to produce. On the other hand, we did not find suppo rt for the condition-dependent hypothesis of sexual advertisement, suggesti ng that the badge may not be a costly trait to produce. However, badge size could reflect other aspects of condition. The kind of pigments involved in colour signals may be the key factor determining the production costs of s uch traits.