Why do you think Dickinson would describe the bird's eyes as "frightened beads"? Is this a simile or a metaphor

Once we read that, the bird,"glanced with rapid eyes/ that hurried all around", we are ready for the next line, "They looked like frightened Beads." We have a good feeling of the bird as a scavenger and even a predator. They eyes give us a sense of a hunter with pinpoint accuracy. The line in question is a simile.