Low genetic diversity in a population of Tricolored Blackbird (Agelaius tricolor), a species pending Endangered status

Abstract

The Tricolored Blackbird (Agelaius tricolor) is a colonial songbird, found almost exclusively in California, whose total population size has sharply declined over the past century. It is currently under review to be listed as Endangered under both the California and U.S. Endangered Species Acts. Here we assess the genetic diversity of a breeding population in California's Central Valley, comparing our findings with previously sampled conspecific and congeneric populations. First, we genotyped 50 adults at 9 microsatellite loci in our focal population and estimated allelic and Shannon diversity, observed and expected heterozygosity, and the inbreeding coefficient (FIS). Second, we compared our results to those of the one existing study on Tricolored Blackbird conservation genetics and found that levels of allelic diversity and heterozygosity in our focal population were similar to those of 11 previously studied populations. Unlike the earlier study, which found moderately high mean inbreeding coefficients, we detected no evidence of inbreeding in our focal population. Third, we used 7 of the 9 loci to compare the genetic diversity of our focal population with populations of 2 previously sampled Agelaius congeners. We found that allelic diversity, Shannon diversity, and expected heterozygosity in our Tricolored Blackbird population were most similar to those of an isolated Red-winged Blackbird (A. phoeniceus) population in the Bahamas. We discuss possible reasons for the different results from the conspecific study, outline why the collective findings from both studies support the need for protective measures, and urge conservation action to maintain existing genetic diversity and gene flow before ongoing population losses lead to adverse fitness consequences.