Trade-offs between molt and immune activity in two populations of house sparrows (Passer domesticus)

Molt and immune defense are critical activities in which all birds must invest. Because each is costly, wild passerines may have to decrease their investment in one activity if they are to increase investments to others. Here, I studied such molt-immune trade-offs in one neotropical and one north-temperate population of house sparrows (Passer domesticus (L., 1758)). I included two populations in my study to investigate if molt-immune trade-offs in this species are phenotypically plastic or fixed. I expected that if they were fixed, then immune activity, molt, and immune-molt trade-offs would be distinct between populations after they were kept in captivity for 1 year under similar conditions. I found evidence for molt-immune trade-offs in house sparrows. Feather growth was inversely related to cutaneous immune activity to phytohemagglutinin (PHA). Furthermore, feather growth 3 weeks post immune challenge was lower in immune-challenged birds relative to saline-injected controls. However, there was no effect of population of origin on these patterns, or the rate of molt or PHA response at this time of year in each population. Thus, while house sparrows probably do face trade-offs between molt and immune activity in the wild, any variation in these trade-offs between populations are likely plastic responses to different environments.