How Conifer Diversity and Availability Influence the Abundance and Biology of the Red Crossbill

Abstract

In order to understand the distributions and abundances of animals, many environmental factors must be considered, particularly the availability of food resources. Food resources are especially important to nomadic species that move in response to the spatial and temporal availability of these specific food resources that are critical to their survival. An example of such nomadic species is the red crossbill (Loxia curvirostra), which specializes on conifer seeds, a resource that significantly varies both temporally and geographically. Thus, crossbills will move large distances each year to find areas with abundant conifer seeds. While conifer seeds impact the distribution, abundance, and reproductive rate of crossbills, it is likely not the only factor driving these patterns. To truly understand what drives the distribution and abundance of crossbills across North America, further study is needed not only on how external environmental factors such as food abundance affect these patterns, but how tradeoffs among internal physiological processes such as reproduction and survival related processes such as immune function may affect when crossbills irruptively migrate or whether or not reproduction occurs. Historically, research to understand how organisms orchestrate their annual cycles with respect to these costly and conflicting physiological processes has focused narrowly on seasonal breeders that constrain reproduction to times of year when thermoregulatory demand is low (i.e., summer), which provide limited opportunities to reveal how physiological costs of different processes may interact with environmental conditions to influence the evolution of investment strategies. In this study, we are examining how the diversity, abundance, and size of cone crop of conifers influence both 1) the quantity and diversity of red crossbills, as well as 2) their seasonal modulation in investment patterns in reproduction and self-maintenance processes such as immune function in Grand Teton National Park, where crossbills can be found breeding in both summer and winter. Preliminary results from this study have indicated that both conifer diversity and cone crop size affect overall quantity and vocal type diversity of crossbills in Grand Teton National Park, as well as affecting their investment in reproduction and immunity. Overall, results from this study will provide information on how species in general and crossbills, specifically, respond to rapidly changing environments, which has become increasingly important in light of the effects of anthropogenic change.