Northern rough winged swallows and where they nest

Rough-winged swallows photographed along the Okanagan Rail Trail near Winfield. We were on a walk along the repurposed railway when we saw numerous swallows flitting about. Many were flying over the nearby lake, but a good many more were busily entering and leaving well-defined holes in the opposing cliff. The cliff was artificial in the sense that it had been created to provide room for the railway. To the swallows, it was real, and provided an ideal place to raise a family. According to All About Birds , the birds don't actually dig their own cavities but rather occupy and nest in preexisting holes created by other creatures. The substrate that such dens are constructed in are banks or cliffs typically made of soft earth of clay, sand, or gravel. The burrows themselves may have been created by rodents, bank swallows, or kingfishers. The above hole was quite large relative to others I saw birds exiting from and I suspect it may have been made by a l...