Boundaries between vegetation types, known as ecotones, can be dynamic in response to climatic changes. The North American Great Plains includes a forest-grassland ecotone in the south-central United States that has expanded and contracted in recent decades in response to historical periods of drought and pluvial conditions. This dynamic region also marks a western distributional limit for many passerine birds that typically breed in forests of the eastern United States. To better understand the influence that variability can exert on broad-scale biodiversity, we explored historical longitudinal shifts in the western extent of breeding ranges of eastern forest songbirds in response to the variable climate of the southern Great Plains. We used climatic niche modeling to estimate current distributional limits of nine species of forest-breeding passerines from 30-year average climate conditions from 1980 to 2010. During this time the southern Great Plains experienced an unprecedented wet period without periodic multi-year droughts that characterized the region’s long-term climate from the early 1900s. Species’ climatic niche models were then projected onto two historical drought periods: 1952–1958 and 1966–1972. Threshold models for each of the three time periods revealed dramatic breeding range contraction and expansion along the forest-grassland ecotone. Precipitation was the most important climate variable defining breeding ranges of these nine eastern forest songbirds. Range limits extended farther west into southern Great Plains during the more recent pluvial conditions of 1980–2010 and contracted during historical drought periods. An independent dataset from BBS was used to validate 1966–1972 range limit projections. Periods of lower precipitation in the forest-grassland ecotone are likely responsible for limiting the western extent of eastern forest songbird breeding distributions. Projected increases in temperature and drought conditions in the southern Great Plains associated with climate change may reverse range expansions observed in the past 30 years.

1980-2010 presence data were collected from eBird basic dataset and the North American Breeding Bird survey (BBS). BBS presences from the first ten stops of BBS routes 1980-2010 were included. eBird presences from checklists submitted that were less than 8 km in distance, less than one hour in duration, and had fewer than 10 observers from 1980-2010 were included. Presence data from eBird and BBS were then spatially rarefied at 40-km using SDM toolbox in ArcMap. (e.g., merg_acfl_1980_40km_rarefied_points.csv)

We also include here BBS presence data 1966-1972 from the first ten stops of BBS routes. The presence data were filtered to only include unique locations; if a species was detected within the first ten stops of a BBS route over multiple years, only one record at that location was used to calculate omission error and the continuous Boyce index. (e.g., BBS_1966_acfl_OE.txt)

Climate data used to train climatic niche models for the nine songbird species and project species distributions onto historical climate conditions are included here. Data include average conditions for the periods 1980-2010, 1966-1972, and 1952-1958. Variables include summer potential evaporation (e.g., d_es3_1952.asc), spring potential evapotranspiration (e.g., dsp_es3_1952.asc) calculated using the Thornthwaite equation, annual precipitation (e.g., p_es3_1952.asc), and average annual temperature (e.g., t_es3_1952.asc).