An exploratory comparison of region-specific feather microstructure in two Eurasian tree sparrow populations across altitudes

High-altitude environments impose simultaneous challenges to avian flight and thermoregulation, potentially shaping variation in feather microstructure. We conducted an exploratory comparison of feather microstructural traits across feather regions, sexes, and altitudinal populations in the Eurasian tree sparrow ( Passer montanus ). Using scanning electron microscopy, we quantified vane- and downy- feather traits, including barbule length and density, hamuli number, cilia number, ventral teeth number, and internode length and width. We further complemented these analyses with thermogravimetric and calorimetric measurements of breast feathers. Our results showed that feather microstructure was strongly region-specific: tail and wing feathers consistently exhibited interlocking structures (similar hamuli, cilia, and ventral teeth), whereas back and breast feathers showed nodal features. In contrast, population- and sex-associated variation appeared limited and localized to body-feather nodal architecture: high-altitude (HA) females had longer breast internodes than low-altitude (LA) females, and breast internodes were narrower than back internodes in LA birds but not in HA birds. Thermogravimetric and calorimetric analyses revealed similar breast-feather decomposition profiles between populations, consistent with no major difference in keratin thermal decomposition. Our results suggest that feather microstructure in this widespread species is primarily dominated by feather-region specialization, with preliminary evidence of localized population- and sex-associated variation.