DOI: 10.5061/dryad.k3j9kd559
title: Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry

description: 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.
description: From March 18–24, 2019, we captured overwintering PSJU<i>
 </i>(n = 7) and SCJU<i> </i>(n = 11)<i>
 </i>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
 (T<sub>0</sub>) 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
 (T<sub>30</sub>) 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
 (<em><strong>BL_PhysiologicalData_Dryad.xlsx</strong></em>) 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 (<em><strong>BL_IsotopeData_Dryad.xlsx</strong></em>) for individual raw hydrogen isotope values.  See <strong>ReadMe file</strong> for a brief overview on raw data files, definitions of variables, and other key information.