If you are a Grasshopper Sparrow, deciding where to defend a territory in a huge area of beautiful, perfect grassland habitat, why would you decide to stick close to other territorial males? This seemingly simple question still doesn’t have an answer, despite it being one of the first questions we tackled when initiating research at the Konza Prairie. We now have a good idea of what DOESN’T drive patterns of aggregation, however!
In 2013, Steffanie Munguia tackled this as her REU project, experimentally testing the prediction that aggregation functions to minimize the costs of nest predation and brood parasitism. But we saw no difference between birds in aggregated vs. isolated territories. The next summer, Sarah Winnicki followed up in her REU project where Steffanie left off. This project eventually involved testing predictions of every hypothesis we could come up with, yet the data are consistent with none of the alternatives. Read this paper (Winnicki_etal_2020_Ecology) and learn why and how social and genetic mechanisms cannot explain aggregation! Our best guess is that population declines and/or the massive changes the Great Plains have experienced have affected the fitness payoffs of this behavior.