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Large, dark swallow. Distinctive north of Mexico, where it is the largest swallow. Adult males are dark overall with a purplish-blue iridescence visible at close range. Females are grayer with iridescence on the crown and back, and variable splotchy patterning on the underparts. Immatures lack any purplish color and have a whiter belly. Gives a distinctive liquid gurgling call. In the eastern U.S., nests exclusively in nest boxes and martin houses; also uses natural cavities in parts of the western U.S. and Mexico. Departs breeding grounds in the late summer, locally staging in massive numbers. Winters primarily in the Amazon basin of South America. During migration and winter, identification is complicated by several other extremely similar species of martin. Identification features are not well known; some are best left unidentified.