Painted Buntings (Passerina ciris) that breed in Oklahoma and molt in Sinaloa, Mexico, demonstrate a clear bimodal pattern of stable isotope ratios in their flight feathers. Some birds had a C3 carbon signature in primary 1 (P1, the first feather replaced during wing molt) and a C4 carbon signature in primary 9 (P9, the last primary to molt), whereas other sympatric birds evinced a C4-based diet throughout feather molt. The bimodal pattern of stable isotope ratios in flight feathers suggests that some birds may initiate molt immediately upon arrival in northwestern Mexico (and carry a C3 signature with them from the breeding grounds) whereas others may delay molt (and grow feathers solely from C4 plants of Sinaloa). From 2010 to 2012, we used geologger tags to test whether differences in the timing and route of fall migration movements were related to stable isotope signatures in primary feathers. We analyzed stable isotopes of hydrogen and carbon in P1 and P9 from 25 individuals fitted with geologger tags in two consecutive years. Of these, 60% changed the diet (C3 vs. C4) that was used to grow P1 between years. We also observed variation among individuals in migration routes, wherein birds from the same breeding population differed greatly in their use of molting and wintering locations. However, we did not find a relationship between isotope signatures and the timing or route of fall migration. We speculate that the bimodal isotope signature we observed represents a carryover effect related to local landscapes (grassland or agriculture vs. shrubland) used during the late breeding season and early molting period, and that these effects diminish as molt progresses. If this is the case, there is the potential for breeding-season diet to directly affect plumage quality in this molt migrant.