| Since 1880, the Great-tailed Grackle has expanded its USA range from southern Texas to include most western states. This range expansion has been marked by the species preferring human-modified environments as breeding grounds, an association that reduces nest predation and provides abundant and consistent food supplies for feeding offspring. Cattle feed lots and large-scale dairies provide reliable food supplies for wintering birds. Given continued human population increase throughout large areas of the western USA, the Great-tailed Grackle will continue its range expansion. One consequence of the range expansion is that across much of Arizona, California, and Nevada, two subspecies, Q. m. monsoni from New Mexico, and Q. m. nelsoni from Sonora, have come into secondary contact and interbred. Using morphological measurements from 122 individuals collected during 1999--2001 and 474 museum specimens collected during 1938--2000, it was found the two subspecies separate clearly using the "75% rule". Uncorrected mtDNA sequence divergence between individuals from New Mexico and Sonora, Mexico, ranged from 1.57--2.26%, suggesting the two subspecies have been separated for 0.83 million years. New Mexico birds separated into two separate clades that were present throughout the area of secondary contact. This third group of birds is most likely the subspecies Q. m. prosopidicola, from Texas and Tamaulipas. Morphological measurements and Discriminant Function Analysis separated over 90% of birds in the zone of secondary contact into either subspecies. The mtDNA analysis of 82 individuals showed limited congruence of 57--68% between morphological measurements and haplotype, suggesting morphological techniques have limited utility in determining the origin of birds collected within the area of secondary contact. Regardless of technique used to identify individual birds, the larger monsoni type birds were less common than expected. Given the Great-tailed Grackles' colonial and polygamous breeding system, the smaller nelsoni birds should be at a disadvantage when competing with the larger monsoni birds. It appears the interaction between these populations is more complicated than initially believed, and environmental influences at the colony level will have to be investigated to gain a better understanding of the factors that appear to be favoring the smaller nelsoni birds. |
