North American grassland birds colonized emerging habitat created by expanding agriculture in a pattern of eastward expansions from the mid-1800s to mid-1900s. These birds have been declining, since at least the mid1900s, largely as result of anthropogenic landscape change. Only one bird that now breeds predominantly in southeastern pine savannas is thought to have experienced a concurrent range expansion into this region: Peucaea aestivalis (Bachman’s Sparrow). However, our understanding of the P. aestivalis expansion, and subsequent retraction to the southeastern United States, is largely based on a contemporaneous review of only a subset of historical records from beyond its modern, northern limit. We suggest an alternative explanation for these historical records is that P. aestivalis historically occurred more broadly than was recognized in contemporaneous literature. To evaluate these hypotheses, we reviewed field observations from literature, natural history collections, and eBird to show how P. aestivalis presence throughout eastern North America has shifted since the mid-1800s. To confirm that these findings were not the result of detection bias, we repeated our analysis on a common sparrow species (Spizella pusilla) with a largely overlapping breeding range, but no history of expansion and retraction. We confirm that P. aestivalis expanded its range, but add that prior to that expansion, its historical distribution was broader than commonly acknowledged today. As a result, we identify the northwestern historical limit of P. aestivalis, the Ouachita and Ozark highlands, as a potential source region for an eastward expansion that is consistent with those of other North American grassland birds of the era. We discuss the potential evolutionary and conservation implications of this range expansion on P. aestivalis given our more nuanced understanding of it. Anthropogenic landscape change initially provided additional habitat for P. aestivalis but has ultimately resulted in a reduction of the P. aestivalis distribution.

Dataset collection and processing is described in the methods section of the associated publication.

To generate the final dataset of eBird records, download all Peucaea aestivalis and Spizella pusilla records from the eBird Basic dataset Version: EBD_relFeb-2018 and use the R code in "nhc-ebird-data-processing-19-Jun-2021.R".

Use the csv files of geographic coordinates in the directory maps to recreate the maps in Figure 2.