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Grackle Expansion

Range Expansion of Boat-tailed and Great-tailed Grackles

The "Cassidix" grackles, Great-tailed and Boat-tailed, present an interesting study both of taxonomy and range expansion. The Great-tailed Grackle (Quiscalus mexicanus) inhabits inland second-growth and agricultural habitats from southwestern North America through Central America to the northern and western coasts of South America. The Boat-tailed Grackle (Quiscalus major) is a characteristic species of brackish coastal habitats along the southeastern Atlantic and Gulf of Mexico coastlines of the United States. Both species are primarily resident in their ranges, and have been undergoing dramatic range expansion northward in the 20th Century. Populations of Boat-tailed and Great-tailed grackles in northern recently colonized areas move southward during winter months.

These two closely-related birds, along with the extinct Slender-billed Grackle of Mexico, represent a "superspecies" formerly assigned the genus Cassidix. From 1902 to 1957 ornithologists considered Boat-tailed and Great-tailed grackles the same species, as they are morphologically similar and can be difficult to separate in the field in their limited range of overlap. Adult Great-tailed Grackles have bright yellow irises, are larger than Boat-taileds, and show different iridescence of plumage. Studies have also shown that differences in vocalization and courtship display keep the two forms separate in areas where both occur, leading to the splitting of the forms to full species status.

On the map of Christmas Bird Count results below it is interesting to note the differential spread of these two sister species. Since for much of the span of the Count these were considered one species, only one map is shown. However, the disjunct ranges allow comparisons to be drawn. During the 1940’s and 1950’s we see a rapid expansion of Boat-tailed Grackles northward up the east coast, eventually slowing down at their present breeding limit of northern New Jersey and Long Island. The range continues to slowly spread northward into New England, as well as to slightly more inland habitats. Great-tailed Grackles in the west remained relatively stable until the 1970’s, when this species began a rapid (and continuing) range expansion northward throughout western North America. Great-taileds also have begun to overlap in breeding range with Boat-taileds in eastern Texas, especially as Boat-taileds move slightly inward from the coast. These trends are especially evident when the birds’ CBC range is compared with the geographic coverage of the count circles. While the density of counts has increased dramatically over the century, the geographic distribution has remained relatively stable. In contrast, the CBC range of both Great-tailed and Boat-tailed grackles has greatly increased.

What may be the causes of these expansions? For Boat-tailed Grackles along the East Coast, it may be that relatively mild winters in mid-century allowed this formerly sub-tropical species to survive northward. Human alteration of coastal habitats may have also played a role. In the case of Great-tailed Grackles in the west, the primary factor is probably human—the agricultural conversion of arid prairie and grassland habitats to irrigated farmland. These newly hospitable areas seem to have allowed Great-tailed Grackles to spread rapidly up the major river systems of the west, jumping from river to river as they forge northward.

Geoffrey S. LeBaron

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