Contributions of feather microstructure to eider down insulation properties

Abstract

Insulation is an essential component of nest structure that helps provide incubation requirements for birds. Many species of waterfowl breed in high latitudes where rapid heat loss can necessitate a high energetic input from parents and use down feathers to line their nests. Common eider Somateria mollissima nest down has exceptional insulating properties but the microstructural mechanisms behind the feather properties have not been thoroughly examined. Here, we hypothesized that insulating properties of nest down are correlated to down feather (plumule) microstructure. We tested the thermal efficiency (fill power) and cohesion of plumules from nests of two Icelandic colonies of wild common eiders and compared them to properties of plumules of wild greylag goose Anser anser. We then used electron microscopy to examine the morphological basis of feather insulating properties. We found that greylag goose down has higher fill power (i.e. traps more air) but much lower cohesion (i.e. less prone to stick together) compared to common eider down. These differences were related to interspecific variation in feather microstructure. Down cohesion increased with the number of barbule microstructures (prongs) that create strong points of contact among feathers. Eider down feathers also had longer barbules than greylag goose down feathers, likely increasing their air‐trapping capacity. Feather properties of these two species might reflect the demands of their contrasting evolutionary history. In greylag goose, a temperate, terrestrial species, plumule microstructure may optimize heat trapping. In common eiders, a diving duck that nests in arctic and subarctic waters, plumule structure may have evolved to maximize cohesion over thermal insulation, which would both reduce buoyancy during their foraging dives and enable nest down to withstand strong arctic winds.

Down Feather Structure Varies Between Low- and High-Altitude Torrent Ducks (MerganettaArmata) in the Andes

“…The lack of observable differences in contour feathers may be related to strong constraints of the waterproof capability of this important outer plumage layer in a species that forages underwater. Further investigation is warranted to examine the microstructures of these feathers (D’alba et al 2017), quantify insulative capacity of the plumage (Walsberg 1988), and determine if these patterns between habitats are repeated across the latitudinal range of M. armata subspecies.…”

“…Merganetta armata sampled from the high-altitude study area (Figure 2) had longer, denser down plumage in the pectoral tract compared to the low-altitude study area (Figure 3). Research has shown that differences in down microstructure are related to differences in insulative properties, as long fibers increase the air-trapping capacity of the feather (D’alba et al 2017). While our observed differences may appear inconsequential from feather to feather, a difference of 3-4 barbs and 2mm of length in the average down feather across the whole body would be expected to have substantial effects on the overall plumage of the animal.…”

“…Our understanding of the importance of down plumage between species is limited as few studies have focused on environmental correlates influencing down structure (but see Williams, Hagelin, and Kooyman 2015; Pap et al 2017; D’alba et al 2017). Our findings represent a novel attempt to quantify interpopulation down feather structure between environments along an elevational gradient.…”

“…The methodology applied in this study was adopted because it was cost effective and easily adapted to an unconventional lab space. Other traits worth investigation are barbule density and feather microstructure (D’alba et al 2017) of M. armata down and contour feathers. An attempt was made to measure feather barbule density for this study; however, the power of the microscope used was not sufficient to be able to reliably detect these incredibly minute structures.…”

“…The energetic costs associated with having suboptimal plumage could be substantial in waterfowl, as thermoregulation can account for 28% of the daily energy expenditure (McKinney & McWilliams 2005). Recent years have seen an increase of studies characterizing interpopulation variation of feather structure due to environmental variation (Middleton 1986; Broggi et al 2011; Gamero et al 2015; Koskenpato et al 2016), but few have focused on waterfowl or comparative data describing down feathers among species or populations inhabiting different environments (but see Pap et al 2017; D’alba et al 2017).…”