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

Thomas A. Gavin, Ronald A. Howard, Bernie May
1991 The AUK: A Quarterly Journal of Ornithology  
ABS?R•C?.--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 _<0.004). Differences in allozyme frequencies we
more » ... 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 behavior 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. ' Present address: State 4-H Office, 809 University Drive East, Texas A&M University, College Station, Texas 77843-2473 USA. 602 specific breeding populations using allozyme data. It is apparent that no one method of analyzing this type of data can provide a complete answer to the amount of present or historical gene flow between populations (Slatkin 1985), but that a diversity of analytical techniques should be employed to develop the most thorough description possible before conclusions are drawn. Red-winged Blackbirds (Agelaius phoeniceus) are one of the most numerous and widespread passetines in North America, with breeding populations from the east coast to the west coast and from Alaska to Costa Rica (AOU 1983). Agelaius phoeniceus is subdivided into 14 subspecies in North America (AOU 1957), which are distinguished by length of wings and tails, body size, shape and size of bill, and coloration of the plumage (Van Rossera 1926, Power 1970). An additional 12 subspecies occur in Central America and the Caribbean (Howard and Moore 1980). Northern populations tend to be migratory, while southern populations are not. Redwinged Blackbirds exhibit breeding-site fidelity (e.g. Nero 1956a, b; Howard 1977; Beletsky and Orians 1987), but similar to most birds, Redwinged Blackbirds are capable of long distance movement. Distances between breeding and wintering areas can approach 1,125 km (Dolbeer 1978). Allozymes in Red-winged Blackbirds 603
doi:10.2307/4088101 fatcat:j5jyo2dh2nh7nlb7wjz3sd7aoq