Gray-breasted Martin is the most widely distributed species of Progne in the Neotropics. This species breeds from northern Mexico south through all of Central America, and in South America south to central Argentina. It is resident throughout most of the vast region, although seasonal changes in abundance at some sites hint that Gray-breasted Martin has some as yet undocumented local movements. It also is partially migratory: it largely departs from eastern Mexico after breeding, and the southernmost populations, from Paraguay and central Brazil south to Argentina, also migrate north after breeding. Much remains to be learned, however, about the details of the seasonal distribution of these migratory populations, and the wintering destination of each migratory population.
All martins are sexually dimorphic, but Gray-breasted is the least dimorphic species of the genus. Both sexes are superficially similar to the female of Purple Martin (Progne subis), having have steel blue upperparts, dusky gray throat and breast, and a white belly.
Three subspecies of Gray-breasted Martin are recognized, but the differences between them are slight. One subspecies, described as slightly duller in color, is resident in western Mexico (warneri). The most widespread subspecies, nominate chalybea, occurs from eastern Mexico south to central South America. The third subspecies, macrorhamphus, occurs in southern South America, and differs from chalybea in its slightly larger size. Surprisingly, a phylogenetic analyis of genetic information (DNA sequence data) revealed a major genetic divide within subspecies chalybea, between populations west of the Andes, which are more closely related to other Central American species martins, and those in South America east of the Andes, which are closely related to Southern Martin (Progne elegans). This unexpected result could reflect hybridization between Gray-breasted and Southern martins; subtle plumage differences between Central American and Amazonian Gray-breasted Martins, however, raise the tantalizing possibiilty that the current classification is in need of a complete overhaul.
As is typical of Progne martins, Gray-breasted Martins occupy a wide variety of semi-open habitats, and adapt well to living in towns and villages. They nest in cavities, which can be natural cavities such as old woodpecker holes, but human provided cavities, such as under the eaves of buildings or in drainpipes, are equally acceptable. Gray-breasted Martins are completely insectivorous, capturing flying insects on the wing.