Quantifying the relative importance of direct and indirect effects influencing bird nestling growth

Nestling growth parameters are integral components of avian life-history strategies as they are crucial determinants of individual survival. Although many factors impact on nestling growth, the relative contribution of each one is still debated in the literature. Most studies rely on the assumption that each factor directly affects nestling growth, but indirect effects mediated by other factors are usually the rule in nature. In this study, we present a comprehensive view of both direct and indirect factors affecting nestling growth using the Red-crested Cardinal (Paroaria coronata) as model system. We evaluated the relative importance of different habitat (forest structure), biotic interactions (botfly larvae ectoparasitism, number of siblings, hatching order), and temporal factors (time of breeding) on nestling growth parameters in 278 nestlings of 128 nests by using piecewise structural equation models. We found that botfly ectoparasitism had the strongest direct effect on nestling growth and, in turn, forest structure increased the probability of botfly occurrence. Besides, the interaction between the number of siblings and hatching order influenced nestling growth, indicating that the first and second nestlings had disproportionately higher growth rates in large than in small clutches. Time of breeding also showed a strong positive indirect effect on botfly occurrence, as well as a weak direct positive effect on nestling growth. Our results demonstrate that, under natural conditions, nestling growth is driven by different factors acting not only directly, but also indirectly on this essential life history trait, and that these factors weave a complex web of interrelated variables.