yellow-wagtails are named, in the off-world, for a behaviour that ornithologists have various theories about - temperature regulation, status signalling, parasite disturbance. Here, the tail generates a faint bioluminescent pulse at the tip, and the creature wags it deliberately, at a distance it has calculated to read as interesting rather than threatening. That pulse is also a counting system: a yellow-wagtail pulsing at twelve beats per minute means approximately twelve others are present and attending to the same target. I have not confirmed the upper limit of this ratio and do not intend to. The beak produces a corrosive secretion - not venom, no injection required - that dissolves protein bonds in muscle tissue at a rate slow enough to notice and fast enough to matter; they strip what they want and move on, and what they want is not all of you. The bioluminescent pulse, for what it is worth, is visible through closed eyelids.