It turns out that usually rich-looking, full and fluffy and curled antique ostrich plumes are not single bird feathers, but sewn-together combinations of multiple feathers.
As stated in the 1928 book The Nu-Way Course in Millinery and Hat Design,
The single plume is used very rarely for a trimming. The full, thick ostrich plume is made up of one feather of which the quill is perfect, built up underneath with less perfect feathers to give the required weight. The feathers are bleached and dyed, then starched, and finally curled.
Further, the 1922 book Millinery says,
Willow plumes were very popular a few years ago. Each tiny feathery fiber is lengthened by having several lengths of the same kind knotted to it, a tedious, fine, hand process. The result is a plume with long, sweeping feathers.
The art of creating these so-full plumes is not lost. See Lynn McMaster's costuming and pattern site for her page on developing nice, thick plumes from several thinner ones, then curling them to taste: Joining, Shaping, and Curling Feathers. This will not create a willow plume, but then, I am not sure many of us would like to spend hours knotting tiny bits of feathers.
Read more about trimming with ostrich feathers from these sewing manuals on the VintageSewing.info site: