Migratory birds are known to be able to navigate—to determine their position on the globe and the direction toward their distant goal—even if they cannot perceive any information emanating from the goal. One hypothesis, that of true bicoordinate navigation, claims that birds should be able to sense and use a grid of two natural parameters as coordinates. Some indirect data support the idea that migratory birds can determine their north—south position, and several recent studies have suggested that at least longdistance migrants, including the Eurasian Reed Warbler (Acrocephalus scirpaceus), can determine their east-west position. How they do this remains a mystery. Birds could theoretically detect the magnitude of east-west displacements if they have two clocks, one synchronizing faster to local time than the other. We tested whether this putative “double-clock” mechanism may serve as a navigational tool for detecting east-west position. We captured Eurasian Reed Warblers during spring migration and tested their orientation in Emlen funnels under capture-site photoperiodic conditions. After these northeasterly oriented control tests, we exposed them to a light—dark regime that simulated a 1,000-km eastward displacement to the Moscow region from which we have shown that actually displaced Eurasian Reed Warblers on spring migration compensate for their displacement by orienting northwestwardly. Exposure to the Moscow light-dark regime did not affect the birds' orientation. Our results suggest that light-dark regime effects alone are unlikely to trigger compensation for the longitudinal displacement in long-distance migratory Eurasian Reed Warblers.
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Vol. 127 • No. 4