GRASSHOPPER SPARROW DISPERSAL ECOLOGY
Grasslands are some of the most dynamic ecosystems in the world. Within and between years, grasslands are regularly shaped by both natural and anthropogenic disturbances. Periodic fire, grazing by large ungulates, and climatic variability all work together to dramatically influence the structure and composition of plant and animal communities that make up this ecosystem. Because these disturbances can be patchy and unpredictable over time, grasslands represent a mosaic of habitats that are constantly in flux over short time scales.
The plants and animals that depend on prairie habitats have adapted unique behaviors to deal with the dynamic nature of this ecosystem. For many animals, these behaviors are expressed as high mobility, as showcased in the huge annual movements made by bison, gazelles, rheas, wildebeests, and other large animals. As one of the most mobile taxa, birds are also incredibly itinerant, capable of long-distance annual migrations and plastic settlement decisions that lead to high rates of breeding dispersal within and among years.
The plants and animals that depend on prairie habitats have adapted unique behaviors to deal with the dynamic nature of this ecosystem. For many animals, these behaviors are expressed as high mobility, as showcased in the huge annual movements made by bison, gazelles, rheas, wildebeests, and other large animals. As one of the most mobile taxa, birds are also incredibly itinerant, capable of long-distance annual migrations and plastic settlement decisions that lead to high rates of breeding dispersal within and among years.
The Flint Hills of North America are a verifiable gem of intact grassland, as the largest remaining tract of native tallgrass prairie in the world. In the middle of the Flint Hills lies the Konza Prairie Biological Station, a 3,487-ha piece of tallgrass prairie right next to Kansas State University in Manhattan. It is here that we discovered a Great Plains population of Grasshopper Sparrows (Ammodramus savannarum) displaying an unusual behavior previously unknown to their life history.
In the first year of studying Grasshopper Sparrows, we discovered that a remarkable number of them moved to new territories half-way through the breeding season. To take a deeper look into this intriguing behavior, I worked with Alice Boyle to tackle the ensuing questions below.
In the first year of studying Grasshopper Sparrows, we discovered that a remarkable number of them moved to new territories half-way through the breeding season. To take a deeper look into this intriguing behavior, I worked with Alice Boyle to tackle the ensuing questions below.
What are the patterns of dispersal? In the first paper of this study, we comprehensively described the patterns and spatial scales of within-season breeding dispersal using a variety of field techniques. We found that between 33 – 75% of individuals disperse mid-way through the breeding season, sometimes moving up to 9 km between territories! We also tested a suite of spatial and temporal factors that may influence changes in density of birds on our study plots. We found that seasonal and management effects (like grazing and prescribed fire regimes) alone could not explain these movement patterns.
Why disperse? In our next manuscript, we test multiple hypotheses relating to nest predation and parasitism avoidance in shaping dispersal decisions. Grassland ecosystems are home to a diversity of nest predators, including animals such as foxes, coyotes, badgers, birds of prey, and snakes. In addition to these risks, Brown-headed Cowbirds (Molothrus ater) can also be a menace to songbirds, as they dump their eggs in nests and regularly poke holes and/or eject host eggs or young out of nests. These combined factors can definitely reduce the fitness of birds, but can also affect dispersal decisions.
Why disperse? In our next manuscript, we test multiple hypotheses relating to nest predation and parasitism avoidance in shaping dispersal decisions. Grassland ecosystems are home to a diversity of nest predators, including animals such as foxes, coyotes, badgers, birds of prey, and snakes. In addition to these risks, Brown-headed Cowbirds (Molothrus ater) can also be a menace to songbirds, as they dump their eggs in nests and regularly poke holes and/or eject host eggs or young out of nests. These combined factors can definitely reduce the fitness of birds, but can also affect dispersal decisions.
What determines where they go? In an upcoming paper, we examine the role of food availability in shaping post-dispersal settlement decisions. As one of the main hypotheses explaining animal movements in general, food is thought to be a driving force in shaping where animals choose to settle and raise young. To test this idea, we used a combination of insect sampling and metabolites analyses to measure food availability and foraging rates in this system.
While the above questions focus on studying the patterns of breeding dispersal within seasons, our work has also examined patterns of between-year breeding dispersal in Grasshopper Sparrows within and across populations. At the Konza, ~16-22% of breeding males return year to year, whereas in other populations breeding across their range, return rates of males dramatically differ. Understanding the demographic and ecological factors that drive differences in breeding dispersal patterns across populations is the subject of ongoing work in our lab.
How common is dispersal among grassland birds? While within-season breeding dispersal is clearly very common in our population of Grasshopper Sparrows, we do not know how common this kind of movement really is in birds. There are few published studies on within-season breeding dispersal and even fewer on grassland birds in particular. While there may be limited work on this subject, there is a lot of anecdotal evidence that suggests this movement may be more common than is generally appreciated. In a forthcoming manuscript, we review the current work on within-season breeding dispersal and summarize what is known about this movement across avian species worldwide.
While the above questions focus on studying the patterns of breeding dispersal within seasons, our work has also examined patterns of between-year breeding dispersal in Grasshopper Sparrows within and across populations. At the Konza, ~16-22% of breeding males return year to year, whereas in other populations breeding across their range, return rates of males dramatically differ. Understanding the demographic and ecological factors that drive differences in breeding dispersal patterns across populations is the subject of ongoing work in our lab.
How common is dispersal among grassland birds? While within-season breeding dispersal is clearly very common in our population of Grasshopper Sparrows, we do not know how common this kind of movement really is in birds. There are few published studies on within-season breeding dispersal and even fewer on grassland birds in particular. While there may be limited work on this subject, there is a lot of anecdotal evidence that suggests this movement may be more common than is generally appreciated. In a forthcoming manuscript, we review the current work on within-season breeding dispersal and summarize what is known about this movement across avian species worldwide.
Header Photo © Dave Rintoul