Table 1. Categories used to describe the position of eight different components of a bird’s posture.

We define each category of posture with photo examples in Table SI.

Posture component	Definition	Position categories
Body orientation	Overall orientation of the signaler’s body	Forward-upright Forward-lowered Forward-normal Side-oriented Feet-forward Above the other species Upside-down
Head position	Position of the signaler’s head relative to their body	Forward-upright Forward-lowered Forward-normal Side-oriented Held-back-and-upwards
Wing position	Position of the signaler’s wings relative to their body	Flapping (for birds actively flapping or hovering in the air) Soaring-gliding Spread-outward Raised-upward Partially-spread Closed-flat Closed-held-slightly-out Closed-raised-off-back
Shoulder position	Position of the shoulders (including underwing/upperwing) relative to the receiver	Underwing-forward Upperwing-forward Wing-horizontal (i.e., shoulder forward, flight feathers trailing) Wing closed with shoulder visible Wing closed with shoulder concealed
Bill position	Position of the bill relative to the receiver, and whether it is open or closed	Open-forward Open-upward Open-downward Open-side Closed-forward Closed-upward Closed-downward Closed-side Note: For video segments, we recorded the bill as ’open’ if it was opened at some point during the aggressive signaling.
Tail position	Position of the tail relative to the body and receiver, and whether or not the tail was fanned	Trailing-fanned Trailing-not fanned Raised-fanned Raised-not fanned Down-fanned Down-not fanned Partly raised-fanned Partly raised-not fanned Side-oriented-fanned Side-oriented-not fanned
Feet position	Position of the feet	On-substrate (including ground, water or perch) Tucked-up Extended Partially extended Hanging
Closest point	Closest part of the signaler’s body to the receiver	See body regions illustrated in Fig. 2.