Resource acquisition and allocation in mountain bluebirds (Sialia currucoides)

Abstract

The strategic allocation of resources to maximize fitness is the organizing principle shaping the evolution of life histories. The physiology and behaviour of animals is a manifestation of trade-offs in resource allocation among traits and activities that benefit current or future reproduction. In theory, traits subject to such trade-offs are expected to show negative correlations, but numerous field studies have documented apparent contradictions to theoretical expectations: in some populations, parental effort (allocation to current reproduction) and reproductive performance (i.e., a fitness component) are not negatively correlated to self-maintenance (allocation to future reproduction). One explanation for such findings is that inter-individual variation in resource acquisition can obscure trade-offs by altering the overall quantity of resources individuals are able to invest. Acquisition may influence both the quantity of resources allocated in trade-offs as well as reproductive success. Asymmetries in acquisition may be the product of differences in intrinsic quality (individual phenotypes) and/or environmental factors (territory quality). In a population of breeding mountain bluebirds (Sialia currucoides), I sought to determine how resource acquisition varied, and to identify how it may affect life-history trade-offs and reproductive performance. In my research, I used several lines of inquiry to characterize how resource acquisition varies and affects trade-offs in the study population. First, I showed that offspring quality, a contributor to fitness, is influenced by the types of prey nestling bluebirds are fed by their parents, and that the resources parents provide to their broods vary seasonally and as their nestlings age. Next, I used a short-term manipulation of brood age to find that parents have limited flexibility in their capacity to acquire resources while provisioning broods, which may be due to intrinsic or extrinsic constraints. I then identified relationships among the landscape characteristics of breeding territories (extrinsic factors, potentially influencing acquisition) and the reproductive performance of bluebirds, over an 11-year period. These characteristics, elevation and distance to forest edges, are subsequently linked to differences in microclimate, microhabitat, and parental prey use, providing a partial explanation for why occupancy and reproductive success are spatially clustered and consistent over time. Finally, I used a brood size manipulation to induce changes in parental effort, revealing that acquisition (quantified as both individual and territory quality) influences how mountain bluebirds trade-off one proxy for self-maintenance (energy metabolites) with providing food to their offspring. Interestingly, these effects differed between the sexes. Females generally biased allocation towards current reproduction, increasing provisioning to larger broods, regardless of territory quality; females on low-quality territories subsidized increased provisioning activity by catabolizing stored resources. In contrast, only males on high-quality territories increased parental effort in response to larger broods, and among these individuals, only high-quality males incurred an energy deficit to subsidize this activity. My findings showed that resource acquisition may be critical to understanding why trade-offs differ among individuals, and that both intrinsic and extrinsic limits on acquisition may influence the patterns of allocation we observe in wild populations.