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Dryad

Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry

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Sep 17, 2020 version files 27.73 KB
Sep 17, 2020 version files 29.50 KB

Abstract

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.