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

BACKGROUND: Migratory birds often exhibit within-species variation in migration routes and non-breeding areas, yet the mechanisms shaping these patterns remain poorly understood, particularly in high-latitude breeding populations. Several hypotheses have been proposed to explain why birds follow particular routes: optimal migration theory proposes that routes minimizing time or energy expenditure are favored, whereas the historical contingency hypothesis posits that routes are shaped by past range expansion, sometimes resulting in "suboptimal" migrations. We investigated whether distance minimization or historical contingency more strongly influenced migration routes in high-latitude breeding myrtle warblers (Setophaga coronata coronata), which indirect evidence previously suggested follow a shorter route to the Pacific Coast rather than the core Gulf Coast nonbreeding area. METHODS: We tracked the migrations of six Alaskan myrtle warblers using geolocators measuring both light and atmospheric pressure and inferred nonbreeding areas using hydrogen isotopes for a larger sample of birds breeding in Alaska, British Columbia, and Alberta (n = 167). Additionally, we compared migration tracks derived from light-level data exclusively with those that incorporated atmospheric pressure. RESULTS: Contrary to expectations, all geolocator-tracked birds and most with stable isotope data migrated to the southeastern United States, with just 5% of individuals possibly wintering on the Pacific Coast. Using pressure data allowed us to resolve migration routes and timing more precisely than traditional light-level methods, while also elucidating flight altitude and fine-scale elevational movements. CONCLUSIONS: 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.

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