Unexpected migration patterns in a high-latitude breeding songbird: evidence from multi-sensor geolocators and isotopes

We found that myrtle warblers breeding in northwestern North America migrate farther than previously thought, despite being generally regarded as a relatively short-distance migrant. Our findings contradict previous studies that suggested myrtle warblers breeding in Alaska and northern British Columbia typically follow a shorter migration route to the Pacific Coast. This seemingly suboptimal route-similar to routes followed by the few other songbirds tracked from the region-is consistent with the historical contingency hypothesis, which proposes that migration routes reflect past range expansions. We recommend that researchers conducting geolocation studies leverage tags with barometers, as the additional atmospheric pressure data greatly improved our ability to characterize migration at a fine scale over the full annual cycle.