Often the emblem of the Secretary. Sometimes one feather is the emblem of the Secretary.
The feathers, or quils, are sometimes surrounded by crowns. I don’t know the significance there, but they are already on the Kirkwall Scroll.
Top: Duncan’s Masonic Ritual and Monitor (1866) by Malcolm C. Duncan
Bottom: Detail of Dąbrowski p. 178 “Masonic Symbols”, early 19th century symbol chart
The ‘feathers’ are in fact quills; you will note that they have been cut to form nibs.
Agreed, ‘feathers’ used for writing.