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In the spring, the same migrants that have overwintered in Mexico remigrate northward to the southern United States to initiate the northern leg of the migration cycle.  Spring remigrants also use an antenna-dependent time-compensated sun compass to direct their northward flight (Guerra & Reppert, 2013).  Fall migrants premature exposed to overwintering-like coldness reverse their flight orientation to the north. The temperature microenvironment at the overwintering site is essential for successful completion of the migration cycle, because without cold exposure, aged migrants continue to orient south. The discovery that coldness triggers the northward flight direction in spring remigrants underscores how vulnerable the migration may be to global warming and overall global climate change.