Individual variation and environmental context in avian life-history evolution: An experimental approach using mountain bluebirds (Sialia currucoides).

Reproductive success of organisms is influenced by life-history traits such as growth and reproductive effort, yet little is known about the proximate factors that influence the allocation of resources to different traits, and how these factors interact. I experimentally investigated the influence of several proximate factors on investment in life-history traits of juvenile and adult mountain bluebirds (Sialia currucoides), an altricial, cavity nesting passerine. Supplemental carotenoids enhanced immune response of nestlings, and allowed nestlings that were parasitized by blow fly larvae (Protocalliphora spp.) to gain body mass more quickly compared to parasitized nestlings that did not receive supplemental carotenoids. When parasites were removed from nests, however, no benefits of carotenoid supplementation were evident. This indicates that, although trade-offs between growth and immunity in nestling birds are functionally driven by limitations in the availability of dietary nutrients, these trade-offs are mediated by ectoparasite exposure. Within broods, supplemental carotenoids enhanced immunity of all but the last-hatched nestling, suggesting that this junior nestling may represent a low-cost resource for parasites. Removal of ectoparasite from nests, however, disproportionately benefited nestlings positioned in the middle of the size hierarchy at hatching, suggesting that parasites balance host defense against nutritional value. In many species, annual reproductive investment may include multiple breeding attempts within a season. In each attempt, females may also select social and/or extra-pair mates. Simultaneous manipulations of timing of hatch of first clutches and food availability throughout first breeding attempts showed that female bluebirds were more likely to double-brood when food availability was high, and that the probability of younger or lower-quality females double-brooding depended on the timing of first breeding attempts. Enhanced food availability in first broods also reduced the probab