Grassland birds have a reputation for being less predictable than their relatives living in forested environments. What kinds of movements do they make? Why do they make them? Why do some populations and individuals exhibit high site fidelity while so many others are never seen again?
Mid-continental grasslands are some of the most variable environments on earth. Within and between years, conditions bird experience can vary tremendously because grasslands are maintained by multiple natural disturbance processes. Fire and grazing by large ungulates have shaped this system for millennia, and along with high variability in annual rainfall, vegetation structure and prey communities are patchy mosaics in space and time. It is hardly surprising then that grassland-dependent birds are incredibly mobile, combining annual migrations with flexible settlement decisions that lead to high rates of breeding dispersal within and among years. We focus on Grasshopper Sparrows for this work, a species in which some populations have breeding-season return rates of over 70%, and others of 0%. At the Konza Prairie Biological Station, 16-22% of breeding males return from year to year, allowing us to (a) determine the individual-level correlates of site fidelity, and (b) examine the drivers of inter-annual variation in this behavior.
In our very first year studying these sparrows, we discovered that a remarkable number of them dispersed to new territories mid-way through the breeding season! Emily Williams tackled this pattern during her MS research, (a) comprehensively describing the patterns and spatial scales of within-season dispersal (Williams & Boyle, 2018), (b) testing a predation-risk avoidance hypothesis to explain which individuals remained site-faithful and which dispersed, and (c) testing the role of food availability in shaping the post-dispersal settlement decisions.
Emily’s MSc thesis also tested hypotheses to explain why some birds disperse within season and others do not (Williams & Boyle, in review), and whether food explained where birds chose to move to, post-dispersal. Meanwhile, we have also been determining the spatial scales of between-year dispersal movements for Konza-breeding Grasshopper Sparrows using stable isotopes, and discovering some remarkable things about site fidelity, birds skipping years but then returning, and wintering locations. There are many more mysteries to figure out in this system!