In a brief Google search I couldn't find any reference to ants using feathers. There's plenty about birds using ants for feather maintenance. Then I found your blog with a very welcome Q&A link. So here goes...
A friend posted on Facebook her observations of the curious actions of a garden ant in her yard. She lives in Santa Fe NM (elev. 7000'). Image and text follows. Could you provide any clues to the behavior demonstrated?
--Is the feather to be used as nest material? Considering the season perhaps that
might be insulation against the coming winter temperatures at that altitude.
--Or is it a structural part of its nest...light weight enough to tote but strong
enough, perhaps with other items to provide some support as needed? If so...clever ant!
--Or do the ants chew feathers and use for some sort of building material? Like wasps?
--Is this widespread (or even known) behavior among this and/or other species of ants?
--Is it driven by elevation and associated climates?
--Or is it simply a decorative item? Is this the Martha Stewart of hippie ants?
Joan (and her friend in Santa Fe, NM)
That you for writing the Ask Ant Q&A.
There are a few ants known to adorn their nest entrances with small feathers, among other similar sized objects such as pebbles and plant fragments. These would fit your second suggestion of using feathers as a structural part of the nest. The picture at this link provides an excellent example of this: https://ces.iisc.ernet.in/thresi/photoAlbum/html/antNests.php?pageNum=28
As for your other hyptheses, I am tickled by the feather as a Martha Stewart-like accoutrement, but I believe in this case that the ant was attracted to the feather by small remains of the bird's tissue or body fluids, and was exerting an extraordinary effort to take home this nutritional treasure to its family.
James C. Trager of the Ask Ant Team