Linked paper: Patterns and correlates of within-season breeding dispersal: A common strategy in a declining grassland songbird by E.J. Williams and W.A. Boyle, The Auk: Ornithological Advances 135:1, January 2018.
Late in the summer of 2013, when Alice Boyle, a new faculty member at Kansas State University, was embarking on studies of grassland birds at the Konza Prairie Biological Station in northeastern Kansas, she noticed something really curious: Individual Grasshopper Sparrows she had color-banded earlier in the season were suddenly popping up in new places, singing their hearts out in locations far from where they were originally captured. Whether this was a weird one-off or a predictable and common behavior of grassland birds, Boyle didn’t know.
I started at Kansas State University as Alice’s first graduate student the next fall. She told me the story of the Grasshopper Sparrows and the strange things they did over the summer. With my fondness for movement ecology and a taste for novelty, I opted to base my master’s thesis on the rogue Grasshopper Sparrows seeking greener pastures.
To rewind a little bit, I should explain why this kind of behavior was surprising. The Grasshopper Sparrow is a small, grassland-obligate migratory songbird that spends its winters along the Gulf Coast and Northern Mexico and travels to the Great Plains to spend its summers. Migration is energetically costly, requiring a lot of time and preparation. Once birds arrive at breeding grounds, they have a relatively short window of time to set up a territory, find a mate, build a nest, raise young, and feed fledglings, and then undergo molt, feed, and prepare for the long journey back to the wintering grounds. All of this has to be accomplished in a span of a few short months. Given the constraints of time, resources, and energy, you’d think that they would stick pretty close to their original territory for the whole breeding season. That is what most migrant birds do, after all. The fact that Grasshopper Sparrows would switch territories, duplicating their efforts of setting up another territory, finding a potential new mate, and trying to nest again—it seems like it wouldn’t be worth it. The fact that Grasshopper Sparrows are indeed doing this—changing territories once, twice, maybe three times, even—makes them apparently unusual compared to their migratory counterparts and begs the question, why go to all that effort?!
Before we could examine why Grasshopper Sparrows move around during the breeding season, we first needed to determine just how common this kind of behavior was. We also wanted to find out the distances over which they traveled, where were the new places they chose to settle, and how frequently they moved locations. Following that initial season in 2013, we set out to answer these questions and looked for this behavior in full force. In the next three seasons of field work, we banded 779 Grasshopper Sparrows, outfitted 19 individuals with radio-transmitters to follow their movements, and searched for color banded birds throughout our study area every week to keep track of territory holders and their whereabouts throughout the season.
What we found, we couldn’t have predicted: Within-season breeding dispersal behavior in Grasshopper Sparrows was way more common than we expected. Depending on which of the different metrics we calculated, between 33% and 75% of males disperse at least once within a single breeding season. The scale of movement between territory locations was also remarkable; one individual moved 9 km between breeding attempts—a movement considered pretty large to a bird that defends an average territory size of 43 meters in diameter! If we had not been systematically looking for this behavior, we might have easily missed it; in many areas, densities of Grasshopper Sparrows remained constant throughout the breeding season, but the identities of territory holders changed, sometimes more than once over the summer.
The fact that these birds are moving around a lot during the breeding season introduces its own list of new questions. Now that we detailed the patterns of this behavior, we could begin to answer the questions of why. Why do they do this? What determines why some leave, and some stay? And what determines where they settle next? Could this be a common strategy of other birds occupying similar habitats? While trying to determine whether this movement was truly unusual by digging into the literature, I actually found quite a bit of evidence for such movements. While the terminology is not consistent, it seems that within-season breeding dispersal could be more common in grassland birds than elsewhere.
The answers to some of these questions formed the rest of my MS research, and some remain as ones we are still working on. But now that the first piece of the puzzle is in place, the next steps are to explore the evolutionary and ecological causes of within-season breeding dispersal in such an interesting little brown job.
To find out more regarding this Grasshopper Sparrow movement story, visit aliceboyle.net and follow us on social media.
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Youtube channel: youtube.com/channel/UCiQiNb9syQ5F455XielMjDA