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In spring, songbirds undergo physiological changes such as migratory fattening and gonadal recrudescence in response to increasing day length. Past research suggests that the day length required to initiate physiological changes, known as the photoperiodic threshold, can vary by breeding latitude. In this study, we explored whether migrants breeding at higher latitudes require longer days in spring before physiological changes occur (i.e., whether breeding latitude of origin predicts photoperiodic threshold). We caught and housed male migrant and resident dark-eyed juncos (Junco hyemalis) in an indoor aviary. Photoperiod was increased incrementally from nine to sixteen hours over fourteen weeks. During each photocycle, morphological measurements of mass, subcutaneous body fat, and cloacal protuberance were measured as indicators of migratory and reproductive condition. Stable isotope signatures of hydrogen were used to estimate breeding latitude as an index of migratory distance. Our results show that migrants and residents differed in physiological changes, as migrants accumulated more subcutaneous fat, increased body mass, and displayed a significant delay in gonadal recrudescence relative to residents. Additionally, individuals breeding at higher latitudes deposited fat at a faster rate than individuals breeding at lower latitudes. These results supported our hypothesis that migratory strategy and breeding latitude may predict differences in photoperiodic threshold for both migratory and reproductive timing. Our findings contribute to the understanding of regulation of timing in annual cycles and improve predictions of how species might respond to changing environments.
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