from The Singing Life of Birds by Donald Kroodsma

The ice ages and the habitats they altered had divided continental (bird) populations, pushing some to the west, some to the east, and in isolation they slowly transformed. .. I see fracture zones, north to south, separating western and eastern wood-pewees, western and eastern kingbirds, red-shafted and yellow-shafted flickers, lazuli and indigo buntings, black-headed and rose-breasted grosbeaks, spotted and eastern towhees, western and eastern meadowlarks, Bullock's and Baltimore orioles, and many more.