Food Availability Cannot Explain Dispersal and Settlement Decisions of a Grassland Bird

Food abundance is often postulated to shape animal habitat selection and movements. Those behaviors are critical to life-history variation and making informed management decisions. However, the importance of food in shaping animal behavior is more often assumed than empirically tested. In mid-continental grasslands, Grasshopper Sparrows (Ammodramus savannarum) display remarkable rates of breeding dispersal, with many individuals sequentially occupying distinct territories within a single breeding season. We tested the hypothesis that within-season breeding dispersal and the iterative settlement decisions resulting from those movements are shaped by spatio-temporal variation in food abundance. Working in an experimentally managed tallgrass prairie in Kansas in 2013-2015, we collected arthropod sweep samples as a measure of food availability (2014) and measured plasma metabolite concentrations to assess short-term foraging dynamics and management of energy stores. We related those metrics to Grasshopper Sparrow territory densities and individual settlement patterns. As predicted, arthropod prey abundance varied seasonally. However, variation in prey did not correlate with metabolite concentrations, and neither prey nor metabolite concentrations were related to sparrow territory densities. Furthermore, metabolite concentrations of sparrows measured following dispersal were not elevated relative to those of sedentary individuals. Our results refute the hypothesis that food abundance explains the behavioral choices of a highly mobile grassland bird and implies that breeding dispersal is shaped by other ecological drivers. Our results underscore the need for critical tests of food-related hypotheses regarding the causes of animal movements and habitat selection. The importance of doing so is especially urgent in rapidly declining species, for which effective management is critical.