Seasonal changes in plumage density, plumage mass and feather morphology in the world’s northernmost land bird

AbstractThe Svalbard ptarmigan is the year-round resident terrestrial bird in the high Arctic. While the physiological and morphological adaptations permitting its winter endurance are reasonably well understood, it remains unknown how the conspicuous moult from a greyish brown summer- to a white winter plumage, and any underlying changes in plumage structure and feather morphology, contributes to seasonal acclimatisation. Thus, using standard morphometric techniques, we, firstly, measured seasonal change in plumage and feather characteristics in six body regions. We then investigated if winter plumage traits differ between first-winter and older birds, because differential plumage acclimatisation has been suggested as an explanation for why young Svalbard ptarmigan lose more heat in winter. Plumage feather density (i.e., feathers × cm-2) and mass density (i.e., mg feathers × cm-2) was higher in winter, particularly on the head and feet where individual feathers were also heavier, longer, and downier. Seasonal changes in other regions (back, tarsi) indicated acclimatisation primarily to resist wear and wind. First-winter and older birds had similar feather density. However, mass density in the young birds was significantly lower in all but one body region (back) since individual feathers weighed less. This can explain previous observations of higher heat loss rates in first-winter birds. Our study suggests that plumage acclimatisation contributes to optimising winter phenotypes, both through higher insulative capacity and by improved resistance to harsh weather. The extent of these adaptations may be balanced by the time or energy available for feather growth, exemplified here by inferior insulation in first-winter birds.