The Cave Swallow is undergoing a general expansion in breeding range and at the same time it is becoming an expected fall vagrant in the northeast of North America. Traditionally there were two populations, represented by various subspecies, one which bred in northern Central America and Mexico, and another which bred in the Greater Antilles. Both of these populations have moved north and are now breeding in the United States, the Mexican birds now commonly found throughout Texas and other parts of the Southwest; the Caribbean birds now nest in southernmost Florida. This swallow is very similar to the Cliff Swallow, but it can be differentiated by its fully rufous throat that extends as a partial collar on the neck; the rufous clearly delineates a blackish cap; this dark capped look is typical of Cave Swallow and not as clearly noticeable on the Cliff Swallow. Furthermore the forehead of Cave Swallows is rufous like the throat, but beware that southwestern United States populations of Cliff Swallows also show rufous on the forehead. It is highly colonial as Cliff Swallows, but the nests are an open cup mud nest, rather than the sphere with side entrance of Cliff Swallows. At least during the initial spread through Texas, some Cave Swallows hybridized with Barn Swallows; this hybridization appears to have lessened in recent years. It is still unclear exactly where Cave Swallows winter, although it appears to be in parts of Central America and southern Mexico, some are observed moving north along the Pacific lowlands of Guatemala, suggesting that many are south of there. Their presence in South America is still unclear. During late fall a recent, during the last dozen years at least, has occurred. At this point it is now expected that in migrant traps along the Atlantic Coast or even Great Lakes young individuals of Cliff Swallows show up. Why this is occurring and why it did not occur historically is a mystery.