Abstract: Pleistocene glacial cycles have been attributed a major role in modifying the distribution and community structure of passerine birds over the last two million years, yet their impact on evolutionary processes such as speciation and the evolution of migration remains controversial. I use patterns of mitochondrial DNA variation, ecomorphology and behavior to test the role of Pleistocene glacial cycles in driving speciation and the evolution of long-distance migration in North and Central American populations of the yellow-rumped warbler ( Dendroica coronata) complex, the chipping sparrow (Spizella passerina), the yellow-eyed junco (Junco phaeonotus), and the dark-eyed junco (Junco hyemalis) complex. Patterns of mtDNA sequence variation in these taxa reveal a long history of isolation and allopatric differentiation during the Pleistocene. Migratory populations in temperate North America are the result of postglacial expansions from ancestral sedentary populations in Mexico into formerly glaciated areas to the north following the last glacial maximum (18,000 years before present). Expanding populations evolved migratory behavior as they colonized temperate regions of high seasonality in the temperate and boreal zones. Glacial isolation and postglacial expansions have impacted different species to remarkably different degrees. Sedentary and migratory chipping sparrow populations show no substantial phenotypic differentiation, whereas sedentary yellow-rumped warblers have rapidly differentiated into the phenotypically divergent myrtle and Audubon's warblers. Remarkably, the postglacial expansion of Mexican yellow-eyed juncos into temperate North America has led to its rapid speciation into at least six distinct dark-eyed junco lineages. An ecomorphological study of migratory and sedentary forms of the yellow-rumped warbler complex revealed that migrants have a pointier, more concave wing shape than sedentary populations, which is consistent with the role of selection in providing migrants with a high-aspect-ratio wing for fast, sustained flight. Based on the genetic and morphological data presented here, I make recommendations regarding the taxonomic treatment of the taxa studied, and propose that several forms currently recognized as subspecies be assigned species rank.