Allozyme Variation among Breeding Populations of Red-Winged Blackbirds: the California Conundrum

We examined allozyme variation in 10 breeding populations of Red-winged Blackbirds (Agelaius phoeniceus) across the United States, which represented samples from 9 of 14 putative subspecies in North America. Variation at 13 of 28 resolvable loci revealed a high level of genetic similarity for all seven populations from Florida and New York through the Great Plains to Oregon and northeastern California (pairwise Nei's distances, all less-than-or-equal-to 0.004). Differences in allozyme frequencies we found suggest that fewer subspecies exist in the continental United States than are currently recognized. The most interesting result was that the genetic distance between the populations sampled at Sacramento and San Francisco Bay national wildlife refuges, which are only 214 km apart, had a Nei's distance approximately 10 times as great as the genetic distance between Florida and Oregon populations. Salton Sea, California, the remaining population sampled, was also highly differentiated. Strong site fidelity, the nonmigratory behaviour of populations at Salton Sea and San Francisco, or both probably explain their relative allozyme distinctness, but the possibility that the brackish environment in which these birds live enforces a selective regime that reduces successful immigration or emigration to other habitats is intriguing.