Greavers are Silkwings, but wear a mask that presumably mutates them into bats
This mask has an eye sigil. Tortodrones have eye sigils. The Swarm is denoted by an eye sigil.
Make your own conclusions here, folks, because last time i made them myself i was met with ridicule. But mockery will not quell the truth.
Actually Greavers are just horrible creatures that have no right to exist, mask-wearing or otherwise.