I read, that feathers did probably bear symbolic/spiritual meaning in the norse as well and they apparently used them as bedding in burials, but of course also stuffed pillows with them.
Paying with feathers is also mentioned in "Historiae adversus paganos" it seems. Can it be assumed they payed in coverts rather than down or is there no indication regarding that?
Now, I also saw it mentioned that they might have been used for decoration purposes on women's clothing and also headdresses, but I couldn't find a credible source for these last claims and wanted to ask if anybody here knows wether this is true or just made-up/speculation.
In their mythology feather cloaks seem to be of importance as well. Did some important people (like goðis for example) maybe wear them as well or is this just reduced to their myths and sagas?
It is perhaps also important to distinguish which species as well as parts of the bird the feather belonged to.
As for the historical use/ trade of feathers (especially downs), first of all, check the following videos on Youtube [in spite of the ban of this subreddit on the specification of using videos as sources]:
NB: Don’t worry (no nsfw), since no killed wild bird appears in the linked videos.
Eiderduck downs mentioned in the videos is usually taken from the nest, left by fledglings. In modern Norway, "Nest house/ box" is often employed to make it easy to collect clean downs.
OP is indeed correct in the identification of Old English additional section to Historiae adversus paganos, so-called Ohthere's account [on King Alfred]. The following is an excerpt, and one of the very few seminal articles on this topic, [Berglund 2009], claims that "bird feathers" whose collection [as tribute] mentioned in the text was probably mainly this kind of down by (wild) eider ducks (Somateria mollissima) - since Old English, source language of Ohthere's account, had perhaps not had the counterpart of the Old Norse word dún ("down") (Berglund 2009: 128).
The following is an excerpt of the account in modern English translation:
"But their wealth consists mostly of the tribute [I prefer to translate gafol primary as such to "the tax" (by Bately)] that Finnas [usually identified with the hunter-gathering Sámi people] pay them. The tribute consists of animals' skins and of birds' feathers (fugela feðerum) and whale's bone and of those ship's ropes that are made from whale's [or walrus'] hide and from seal's. Each pays according to his rank: The Highest in rank has to pay fifteen marten's skins and five reindeer's and one bear's skin and ten ambers [unit] of feathers and a bear- or otter-skin tunic and two ship's ropes: each must be sixty ells long, one must be made from whale's [or walrus] hide, the other from sealskin (Bately ed. & trans. 2007: 46)."
The documentary evidence of the collection of eiderdown duck in northern coastal Norway certainly dates only back to the 18th century, but Berglund supposes that its history dates further back into the Middle Ages - skerries (sea shore) on which wild ducks often nest in northern Norway had often been owned by the archbishop of Trondheim/ Nidaros, the most powerful landlord in northern part of Norway, according to the 15th century land register.
Quilts and pillows (cushion) filled with such eiderduck down or other waterfoul's feathers (such as mallard duck, domesticated gooses) are now sometimes identified among Iron Age and Viking Age archaeological finds (Berglund 2009; Berglund & Rosvold 2021; Panagiotakopulu, Buckland & Wickler 2018).
On the other hand, feathers of other species of birds like owls or swans, located sometimes near the helmet or the hut, are also recently identified with the use of microscopes, and Berglund & Rosvold suggest these might had something with the possible ritual use of feathers (though the extant written evidence or folklore might be too inconclusive, I'm personally afraid).
Anyway, the deeper exploration of this topic by scholars has apparently just began since the last decades of the 20th century (with help of better zooarchaeological scientific tools like microscopes), so we'll wait for further research.
You might also be interested in my previous answer in Are jewellery with crow feather common for vikings raiders?
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