Fieldwork and sampling
We went on two expeditions in 2016 and 2017 to acquire genetic samples from populations of several bird species on both sides of the Great Rift Valley (GRV) (Fig. 1). We mist-netted birds and sampled: (1) blood, (2) wing measurements, and (3) photographs of each individual. All fieldwork was done in accordance with NYU IACUC accepted procedures (IACUC Protocol Number 15-00002A1 to SB) and permissions from the Ethiopian Wildlife Conservation Authority and the Oromia Forest and Wildlife Enterprise. For this study, we included six species from which we captured individuals on both sides of the GRV (Table 1). Of note, we were limited to three samples per species per region because of permitting regulations.
We sampled a total of ≤ 20 μL of blood from each individual for genomic resources. As a proxy for dispersal ability, we took measurements of the (1) wrist to tip of longest primary and (2) length of first secondary feather to calculate the hand-wing index (HWI) for each individual (Claramunt et al., 2011; Kipp, 1959). Lastly, we took profile photographs of each bird. Information of genetic samples, bird photos, locality information, and habitat photos are accessioned at the University of Kansas Biodiversity Institute, with much of the information presented in Table S1.