Geographic shifts of phoebe species (Sayornis spp.) in the central Great Plains of North America

Climate and vegetation change can affect interactions among bird species, and may result in shifting range boundaries. Long-term interspecific range dynamics can be explored in hybrid zones or contact zones to evaluate these shifts. In this study, we document the historical nest site distribution of Eastern (Sayornis phoebe, n = 31) and Say's (S. saya, n = 21) phoebes, and subsequent habitat change over a 48 year span (1973-2021) across a study area in west-central Kansas (Ellis County), in the central Great Plains of North America. We used Normalized Difference Vegetation Index (NDVI) to assess vegetation biomass in relation to phoebe occurrences over this time period. The ratio of Eastern:Say's was 1.5:1 in the early period (1973-1974), compared to 8.4:1 in the most recent period (2020-2021). In the most recent period, the 16 nest sites occupied by Eastern Phoebes, but that had been occupied by Say's Phoebes previously, had significantly increased change in NDVI values, suggesting that habitat changes (i.e. increasing vegetation) favor Eastern Phoebes, and might explain ongoing westward range shifts and geographic displacement of Say's by Eastern phoebes. On a broader (regional) scale, records of hybrid phoebes from citizen science databases have increased recently, mostly Eastern x Black but more rarely Say's x Black, suggesting a dispersed expansion of hybridization in the southwestern Great Plains and southern Texas.