Bird identification by behavior is the skill that separates two species that look nearly identical. Watch what a bird actually does and the answer often arrives before the field marks do.
Spotted Sandpipers have a very unique tail-dipping motion that tells you exactly what you're looking at. No other shorebird does it.
Behavior clues worth tracking:
Feeding style. Ground-gleaning, bark-probing, catching insects in midair, hovering at flowers.
Movement. Hopping versus walking, tail-bobbing, wing-flicking.
Perching posture. Upright and alert versus low and horizontal.
Social pattern. Alone, in pairs, or traveling with a mixed flock.
Behavior is also one of the few clues that still works when a bird is backlit or the light is poor. You don't need to see color to see how something moves.
What's a behavior that tells you which bird you're looking at without needing a closer look?