Quantifying population genetic heterogeneity within nonbreeding aggregations can inform our understanding
of patterns of site fidelity, migratory connectivity, and gene flow between breeding and nonbreeding areas.
However, characterizing mechanisms that contribute to heterogeneity, such as migration and dispersal, is required before site fidelity and migratory connectivity can be assessed accurately. We studied nonbreeding groups of Common
Mergansers (Mergus merganser) molting on Kodiak Island, Alaska, from 2005 to 2007, using banding data to assess
rates of recapture, mitochondrial (mt) DNA to determine natal area, and nuclear microsatellite genotypes to assess
dispersal. Using baseline information from differentiated mtDNA haplogroups across North America, we were able to
assign individuals to natal regions and document population genetic heterogeneity within and among molting groups.
Band-recovery and DNA data suggest that both migration from and dispersal among natal areas contribute to admixed
groups of males molting on Kodiak Island. A lack of differentiation in the Common Merganser’s nuclear, bi-parentally
inherited DNA, observed across North America, implies that dispersal can mislead genetic assessments of migratory
connectivity and assignments of nonbreeding individuals to breeding areas. Thus multiple and independent data types
are required to account for such behaviors before accurate assessments of migratory connectivity can be made.