Landscape change promotes population differentiation at a small geographic scale

Landscape change can expose individuals to rapidly changing conditions, and thus may accelerate phenotypic divergence among populations. However, the spatial scale at which phenotypic change can occur remains poorly understood. Here, we used a 5-year (2008-2012) database to examine morphological differentiation and site fidelity of red-necked nightjars (Caprimulgus ruficollis) breeding in two nearby (10 km) habitats subjected to contrasting management practices: a mosaic landscape of cattle-grazed pastures and tree plantations and a protected reserve where human access and activities are restricted. We also quantified the potential foraging opportunities for nightjars at both sites, measured by aerial prey abundance, availability of foraging habitats and their proximity to nesting sites. Nightjars were not randomly distributed with respect to morphology. Breeding birds from the managed area were significantly larger (keel length) than those from the natural reserve, whereas there were no differences in wing or tail length. No individual (out of 1,130 captures overall) moved between habitats over the study period, indicating strong site fidelity. Food availability was similar in both habitats, but the availability of foraging sites and their proximity to nesting sites were found to be highest in the managed property, which likely translated into enhanced access to food in the latter area. Taken together, our results suggest that landscape change underlines the observed fine-scale variation in nightjar morphology, since the increased proximity between foraging and nesting sites in the managed area leads to increased food intake and growth rates of nestlings and, as a result, they likely attain larger adult sizes in relation to those in the natural site. High site fidelity probably contributes to maintain the body-size differences between the two habitats. These results come on top recent studies highlighting the role of habitat alteration by humans in promoting population differentiation, even at very small spatial scale