Why birds should communicate by dynamic optical signal patterns and not by the static signals perceived by man

Abstract

The ability to fly is a prominent feature of birds. While the changes in morphological structures during motion are well-known, the dynamics of optical signals generated by the birds' plumage are hardly perceived. Most studies and illustrations focus on static situations, with birds perching, or flying with their wings constantly spread. But given birds' capability for active flight, two assumptions seem to be imperative: birds should show dynamic signal patterns alternating periodically with the wing-strokes; birds should rely much more on dynamic signalling than on static marking. When spreading and folding the wing (and tail) during active flight, markings contrasting to the rest of the feathers may generate a flashing signal. Contrast may be intensified by “framing”. Thus in a wing stripe that is periodically “unveiled” in flight, lighter areas may be “framed” by darker parts of the vanes or shafts. Perception of dynamic signals is hampered by their complexity because of changes in size, shape, and orientation during motion. The complete pattern may be visible for a few milliseconds only. Dynamic signals may be more conspicuous because of rapid switching on and off. The evolution of contrasting signal patterns may reduce the mechanical stability of feathers. Thus, selection should balance these costs against the benefits of the signal's efficiency. Birds have to be viewed in natural surroundings, on different levels, and in action. A quantitative approach to dynamic optical signals is still largely lacking. Although hidden to human eyes, dynamic signal patterns extending to the UV may be widespread among birds.