biostudies-literatureUnknown16Singh DEvery year as spring approaches and day length increases, many birds begin to reproduce, an annual expression of seasonal phenology that requires physiological preparation. In species distributed over a broad geographic range, populations that breed at higher latitudes are often migratory and delay reproduction until later in the year as compared to those breeding at lower latitudes. Dark-eyed Juncos serve as an excellent model for understanding the timing mechanisms regulating population-level variation in seasonal reproductive responses. We compared two seasonally sympatric dark-eyed junco populations in early spring. One migrates (<i>Junco hyemalis hyemalis</i>) and breeds in Alaska and Canada, while the other remains resident (<i>Junco hyemalis carolinensis</i>) and breeds in the AppalFrontiers in physiology1627516https://www.ebi.ac.uk/biostudies/studies/S-EPMC12440951biostudies-literatureSeasonal divergence in reproductive timing on the verge of spring: comparing hypothalamic transcriptome of two seasonally sympatric North American songbird populations.PMC12440951Fudickar AMKetterson EDSingh DfalseSeasonal divergence in reproductive timing on the verge of spring: comparing hypothalamic transcriptome of two seasonally sympatric North American songbird populations.Every year as spring approaches and day length increases, many birds begin to reproduce, an annual expression of seasonal phenology that requires physiological preparation. In species distributed over a broad geographic range, populations that breed at higher latitudes are often migratory and delay reproduction until later in the year as compared to those breeding at lower latitudes. Dark-eyed Juncos serve as an excellent model for understanding the timing mechanisms regulating population-level variation in seasonal reproductive responses. We compared two seasonally sympatric dark-eyed junco populations in early spring. One migrates (<i>Junco hyemalis hyemalis</i>) and breeds in Alaska and Canada, while the other remains resident (<i>Junco hyemalis carolinensis</i>) and breeds in the Appal2025-01-01T00:00:00Z20252026-04-26T03:20:34.212Z2026-04-26T03:11:50.926ZS-EPMC124409514096992110.3389/fphys.2025.1627516