Effective conservation management of migratory birds can be challenging and requires knowledge about population structure and the strength of migratory connectivity. The latter likely affects the degree to which populations are locally adapted and differentiated, and both may affect their ability to adapt to environmental changes. Western Sandpipers (Calidris mauri) are long-distance migrants that exhibit latitudinal differences in morphology and life-history strategies across their non-breeding range. The differential migration patterns in this small shorebird species could be based on population genetic differences or phenotypic plasticity. I investigated the population structure of Western Sandpipers, and its implications for differential migration and conservation, across their global range using genetic, morphometric and acoustic data. I recorded and analyzed breeding male vocalizations, conducted playback experiments and collected genetic and morphometric data across breeding and non-breeding ranges. Amplified Fragment Length Polymorphisms (AFLP) and mtDNA sequence data indicated low, but significant genetic population structure across breeding and non-breeding ranges. I found evidence for an evolutionarily recent demographic expansion (mtDNA), for latitudinal clines in genetic scores (AFLP) and for isolation by distance (AFLP). A scan of thousands of AFLP markers in a pooled lane approach revealed no fixed genetic differences among juveniles with a postulated difference in life-history strategy or migratory direction. Songs, but not alarm calls, varied geographically, decreasing in length and increasing in fundamental frequencies from southern to northern breeding sites. These geographic differences were sufficiently large to allow males to discriminate between local and non-local songs and hence are potentially biologically relevant. AFLP scores and vocalization frequencies showed correlations with body size. In conjunction, my results suggest that Western Sandpipers underwent a recent range and population expansion that has resulted in divergence patterns that are primarily isolation by distance driven. My results suggest interrelationships among genetic population structure, morphology and latitudinal segregation. While populations were not sufficiently distinct to allow for assignment tests, the latitudinal gradients found across both the breeding and non-breeding grounds are suggestive of a gradual ’chain’-like migration pattern, but do not indicate strong migratory connectivity. Consequently, conservation strategies could focus on the protection of major sites at different latitudes.
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