The initiation of reproduction in many seasonally breeding animals is controlled by photoperiod and tends to be clinal: populations at higher latitudes breed later than those at lower latitudes, often reflecting a higher photoperiodic threshold. Migratory animals presumably time reproduction to match conditions at their breeding grounds at least in part by cues perceived on their wintering grounds. We asked how closely related dark-eyed junco (Junco hyemalis) populations that overwinter in sympatry but breed in allopatry respond to their shared winter environment by comparing early spring indices of readiness to migrate and breed (baseline and elevated testosterone). We measured stable hydrogen isotopes from feathers grown the preceding year and claws grown during winter to estimate breeding and wintering latitudes, respectively. We predicted that if reproductive initiation is adapted to the emergence of resources at their respective breeding destinations, then birds migrating to higher latitudes (slate-colored junco; J. h. hyemalis) should delay breeding as compared to those migrating to lower latitudes (pink-sided junco; J. h. mearnsi) despite a common overwinter environment. We found higher testosterone in pink-sided juncos consistent with earlier reproductive initiation, suggesting local adaptation in reproductive phenology achieved through differential responses to predictive environmental cues.

From March 18–24, 2019, we captured overwintering PSJU (n = 7) and SCJU (n = 11) at three field sites: Chatfield State Park in Littleton, CO (39°31’44”N, 105°05’19”W), Denver Audubon Nature Center in Littleton, CO (39°29’35”N, 105°05’48”W), and Peyton, CO (39°05’36”N, 104°25’29”W).  Birds were caught using baited potter traps and banded with aluminum United States Fish and Wildlife Services (USFWS) bands. All standard morphometric measurements were taken immediately after capture. We measured Individual tarsus length, wing length, mass, body fat, and pectoral muscle condition. Blood samples were collected immediately after capture to measure baseline circulating testosterone (T₀) by puncturing the alar wing vein. Birds then received an intrapectoral injection of GnRH, (1.25 µg chicken GnRH-I, Sigma L0637; American Peptide 54-8-23, Sunnyvale, CA), after exactly 30 min, a second blood sample was taken to measure elevated testosterone (T₃₀) levels. The blood samples were stored at 4°C, processed for plasma extraction and stored at -20°C until analyses.  See detailed procedure for morphometric measurement, physiological, and hydrogen isotope data collection in electronic supplementary methods. Please refer to file named (BL_PhysiologicalData_Dryad.xlsx) for individual bird data. 

Feather and claw hydrogen isotope values were measured to estimate the breeding and wintering latitude of the birds. See detailed procedure hydrogen isotope analyses and statistics details in electronic supplementary methods. Please refer to file named (BL_IsotopeData_Dryad.xlsx) for individual raw hydrogen isotope values. 

See ReadMe file for a brief overview on raw data files, definitions of variables, and other key information.