Seasonal migration and genetic population structure in House Wrens

Neotropical migrant birds have undergone significant changes in population size, due in part to alterations in the landscape in their breeding range. Some alterations have resulted in the isolation of areas containing optimal habitat for breeding. Isolation between habitat fragments could reduce gen e how between populations, which might result in loss of genetic variation due to genetic drift. To test whether isolation due to distance between pop ulations may affect migratory birds as much as sedentary birds, we used fou r microsatellite loci as genetic markers to compare gene flow and populatio n structure in migratory and sedentary House Wrens (Troglodytes aedon and T musculus, respectively). If migratory behavior enhanced gene flow, we expe cted to find that populations of migratory birds were genetically more simi lar than populations of sedentary birds, and that gene flow declined more r apidly with distance in sedentary than in migratory populations. Blood samp les were collected from 18-25 migratory House Wrens at each of six sites in Ohio separated by distances from 25 to over 300 km, and from 16-20 sedenta ry House Wrens at each of six sites in Costa Rica, also at distances up to 350 km. We used heterologous microsatellite primers to obtain estimates for R-ST (population subdivision) and Nm (number of immigrants per generation) in relation to distance between pairs of populations. In the migratory pop ulations, Mantel permutational tests showed no effect of distance on R-ST o r Nm at distances between 25 and 300 km. In the sedentary birds, R-ST incre ased and Nm decreased significantly as distance between populations increas ed from 25 to 300 km. F-ST values were not different from those obtained us ing Slatkin's (1995) R-ST statistics. These results are consistent with our hypothesis that migratory behavior enhances gene how.