Cold tolerance, and not earlier arrival on breeding grounds, explains why males winter further north in an Arctic-breeding songbird

摘要: Sex biases in distributions of migratory birds during the non-breeding season are widespread; however, proximate mechanisms contributing to broad-scale sex-ratio variation not well understood. We analyzed a long-term winterbanding dataset combination with spring migration data from individuals tracked by using geolocators test three hypotheses for observed sex-ratios wintering fl ocks snow buntings Plectrophenax nivalis . quantifi ed relevant weather conditions winter (temperature, snowfall and depth) at each banding site year measured body size condition (fat scores) individual (n � 5500). also directly distance 17 light-level geolocators. If distribution pattern is related interactions between thermoregulation, then larger bodied (males) should be found colder sites (body hypothesis). Males may closer breeding grounds reduce early arrival (arrival timing Finally, males socially dominant over females, thus exclude females high-quality (social dominance support hypothesis, that snowier predicted both higher proportions banded. Direct tracking revealed did signifi cantly their site, despite being slightly further north on average than same population. some evidence social dominance, tended carry more fat males, potentially indicating lower habitat quality females. Global climatic warming temperature constraints smaller-bodied resulting changes distributional patterns. Whether this has repercussions fi tness, therefore population demography, an important area future research. Variation sexes non