The beautiful sack on the neck of a bird, can be a literal sack such as frigate birds and marabou storks, or feathers, such as a ravens hackles, or a tui's neck feathers.
Oh Joshua, I'd love to go stroke a big hairy birdsack one day! Wont you come with?
Why yes Emily my dear, I'd love to play with some birdsacks. Let's go to the Raven Aviary to play with their Big Black Bushy Birdsacks.
a small skimpy bag of the good herb. or when your sack flies away.
Sorry bro all I can get is a birdsack., Damn dog a birdsack just flew by.