Disparity in population structuring of Southwestern Willow Flycatchers based on geographic distance, movement patterns, and genetic analyses

Estimates of population connectivity often are based on demographic analysis of movements among subpopulations, but this approach may fail to detect rare migrants or overestimate the contribution of movements into populations when migrants fail to successfully reproduce. We compared movement data of endangered Southwestern Willow Flycatchers among isolated populations in Nevada and Arizona from 1997 to 2008 to genetic analyses of samples collected between 2004 and 2008 to determine the degree to which these two methods were concordant in their estimates of population structuring. Given that documented movements of 13 color-banded adults and 23 juveniles over 10 years indicated low rates of long-distance movements, we predicted that genetic analyses would show significant population structuring between a northern (Nevada) deme and a southern (Arizona) deme. We genotyped 93 adult individuals at seven microsatellite loci and used two Bayesian clustering programs, STRUCTURE and GENELAND, to predict population structure. Both clustering algorithms produced the same structuring pattern; a cluster containing birds breeding in Pahranagat National Wildlife Refuge, the northern-most Nevada site, and a cluster comprised of all other populations. These results highlight that estimates of subpopulation connectivity based on demographic analyses may differ from those based on genetics, suggesting either temporal changes in the pattern of movements, the importance of undetected movements, or differential contribution of migrants to the subpopulations they enter.