Seasonal changes in plumage density, plumage mass, and feather morphology in the world’s northernmost land bird, the Svalbard Rock Ptarmigan (Lagopus muta hyperborea)

Abstract

The Svalbard Rock Ptarmigan, Lagopus muta hyperborea, is the only year-round resident terrestrial bird in the high Arctic. Many of the physiological and morphological adaptations permitting its winter endurance in this harsh environment are well understood. However, 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, contribute to seasonal acclimatisation. We used standard morphometric techniques to describe the seasonal change in plumage and feather characteristics in six body regions. Then, we investigated whether winter plumage traits difered between frst-winter and adult (second winter, or older) birds, because a diference in coat insulation has been suggested as an explanation for why frst-winter Svalbard Ptarmigan loose more heat than adults. Plumage feather density (i.e., feathers× cm⁻²) and mass density (i.e., mg feathers× cm⁻²) were 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 physical wear and wind. First-winter and adult birds had similar feather densities. However, mass density in frst-winter birds was signifcantly lower than in adults in all but one body region (back) because individual feathers weighed less. This can explain previous observations of higher heat loss rates in frst-winter birds. Our study suggests that plumage acclimatisation contributes to optimising the winter phenotype of Svalbard Rock Ptarmigan, both through higher insulative capacity and by improved resistance to harsh weather. The extent of these adaptations may be determined by the time or energy available for feather growth, exemplifed here by inferior insulation in frst-winter birds.