An exploratory comparison of region-specific feather microstructure in two Eurasian tree sparrow populations across altitudes 1 Hebei Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology, College of Life Sciences, Hebei Normal University, Shijiazhuang, China 2Hebei Collaborative Innovation Center for Eco-Environment, Hebei Normal University, Shijiazhuang, China 3Cangzhou Normal University College of Life Science, Cangzhou, China DOI 10.7717/peerj.21803 Academic Editor...