The study of animal movement ecology is limited by fundamental gaps in our understanding of the nature and individual variability of movement patterns, especially in highly mobile species. These knowledge gaps restrict our ability to explain the causes of movement strategies, and result in demographic inferences and conservation recommendations that may frequently be wrong. Mounting evidence suggests that mid-continental birds are exceptional in their lack of breeding site-faithfulness reflecting their tendency to make breeding dispersal movements within and between seasons. Determining the spatial scales of individual movements is key to determining whether breeding dispersal functions in the context of inbreeding, kin-selection, or local-scale variation in landuse, or rather, is a response to large-scale inter-annual variation in precipitation patterns that ultimately shape habitat quality. I tested key predictions of this latter hypothesis in a quintessential grassland-breeding bird, the Grasshopper Sparrow (Ammodramus savannarum).
In fall 2014, I sampled new, growing feathers, and old feathers grown the previous year l at 28 sites in KS, NE, OK, and MO, distributed in a 400-km radius around the Konza Prairie Biology Station in NE Kansas. In spring 2015, I also sampled feathers from newly-captured birds breeding at Konza. I used δ13C and δD of known-location feathers to construct a year-specific regional map of feather isotopes. While the new, growing feathers exhibited strong geographic gradients in both isotopes, old feathers collected from the same individuals did not. The isotopic signatures of newly-captured birds at Konza in 2015 revealed that they had not bred nearby the previous year, but most likely spent the 2014 breeding season in SW Kansas or N Oklahoma. The directions of those movements is consistent with birds moving from regions experiencing below-normal precipitation in 2014 to areas that had been wetter during that same period. These results are all consistent with large-scale climatic processes accounting for the high mobility of mid-continental bird populations and have important implications for the conservation of this threatened guild.