Historical climate change as driver of populational range expansion and differentiation in a rare and partially migratory Neotropical bird

Populations peripheral to a species’ distribution are more susceptible to new selective pressures, changes, differentiation, and extinction. The Sooty Swift Cypseloides fumigatus (Apodidae) is a rare bird found mainly in central and southeastern South America. Populations of this species are resident in some localities throughout the year, whereas other populations migrate following the breeding season. Recently, a northern population was found well beyond the limits of its known distribution, with approximately 1000 km of occurrence gap between this newly discovered population and the southern distribution. Here, we reveal changes in species distribution over time, as well as compare the morphometric and genetic patterns of populations in the central-southern distribution of the Atlantic Forest and the northern peripheral population within the Caatinga. Our climate niche models indicate a scenario of expanding species distribution during the Last Glacial Maximum, which potentially drove the colonization of the northernmost population. This scenario of peripheral isolation is also supported by the spatial morphometric and genetic variation among populations of the humid forest enclave within the Caatinga and the Cerrado–Atlantic Forest populations. These findings provide important insights into the connectivity between distant Sooty Swift populations, the species' migratory behavior, as well as implications for conservation.