The plant on fol. 46v of the Voynich manuscript has one of the most fascinating roots in the book. D'Imperio describes it as "a bird with spread wings: an eagle!" in An Elegant Enigma. I believe the plant was meant to represent costmary (Frauenminze) - found in the old herbals as the herb of Virgin Mary.
So I looked around to see if I can find any connection in iconography between Virgin Mary and spread-winged eagle. Here are couple of examples.
1491 propaganda brochure by emperor Maximilian I tells the story of celestial vision at Constantinople described as virgin with wings held by three-headed knight. BSN cgm 598, Buch der heiligen Dreifaltigkeit, dated 'not before 1467' shows the Virgin and Jesus crucified in a form of double-headed eagle. Other alchemy manuscripts (15th-16th century) show winged Virgin as symbol of philosophical mercury.
So in late 15th century Germany somebody could have made association between costmary (the herb of Virgin Mary) and eagle with spread wings.