Revealing the demographic history of the European nightjar (Caprimulgus europaeus)

A species' demographic history gives important context to contemporary population genetics and a possible insight into past responses to climate change; with an individual's genome providing a window into the evolutionary history of contemporary populations. Pairwise sequentially Markovian coalescent (PSMC) analysis uses information from a single genome to derive fluctuations in effective population size change over the last ~5 million years. Here, we apply PSMC analysis to two European nightjar (Caprimulgus europaeus) genomes, sampled in Northwest and Southern Europe, with the aim of revealing the demographic history of nightjar in Europe. We successfully reconstructed effective population size over the last 5 million years. Our analysis shows that in response to global climate change, the effective population size of nightjar broadly increased under stable warm periods and decreased during cooler spans and prolonged glacial periods. PSMC analysis on the pseudo-diploid combination of the two genomes revealed fluctuations in gene flow between ancestral populations over time, with gene flow ceasing by the last-glacial period. Our results are tentatively suggestive of divergence in the European nightjar population, with timings consistent with differentiation being driven by restriction to different refugia during periods of glaciation. Finally, our results suggest that migratory behaviour in nightjar likely evolved prior to the last-glacial period, with long-distance migration seemingly persisting throughout the Pleistocene. However, further genetic structure analysis of individuals from known breeding sites across the species' contemporary range is needed to understand the extent and origins of range-wide differentiation in nightjar.