ARE BIRDS DYNAMICISTS?

Maurice N Brearley

We know, of course, that birds are aerodynamicists. They wouldn't be able to fly so well if they didn't know about such things as lift and drag coefficients, turbulence, thermals, ground effect, etc., and they understand relative velocity better than many mathematics students. Whoever saw a bird do a downwind landing?

Many years ago I wondered why many species of birds oscillate their heads forward and backward as they walk. I noticed that they do this with the same frequency as their leg movements, and I wondered if it is simply a reaction to the sudden muscular leg action. If that were so, why don't seagulls also exhibit this head oscillation when they walk?

I wrote to Harry Butler, the wild-life expert, and asked him if he could offer an explanation for the head oscillations. He replied courteously, and said that he thought it was an aid to estimating the distance of objects to the side of the bird. He said that many birds have their eyes on the sides of their heads, and so are virtually monocular when viewing objects which are off to one side; chameleons use such head movements to estimate distances, and he thought that birds may be using the same technique. I doubted this, as the forward movement of walking would itself serve the purpose of distance estimation.

I think there is a dynamical reason for the head oscillations. When feeding, birds pick up from the ground small objects such as seeds, their acute eyesight enabling them to see very tiny objects. When a bird walks, the off-centre thrust of each leg causes its body to rotate slightly. This waddling action, so clearly visible in ducks, is present to some degree in all birds, and it impairs slightly the acuity of a bird's vision. When the bird's head is thrust well forward, the moment of inertia of its body about a vertical axis is increased, and as a result the amplitude of its angular waddle is decreased [1]. This causes the acuity of the bird's vision to be slightly increased, resulting in more efficient feeding. A bird with this characteristic would feed better, and so would live longer and therefore produce more offspring having this same characteristic. In accordance with Darwin's theory of evolution, head oscillating birds would eventually predominate in each ground-feeding species.

To enable me to study bird-walking behaviour more precisely, I asked The University of Melbourne's Centre for the Study of Higher Education for assistance. One of their staff members made a video of pigeons walking in the Botanical Gardens [2]. By viewing this video, in slow motion and frame-by-frame, I was able to discover that the forward thrust of the bird's head occurs very suddenly just at the start of each step (see Figure 1). This is the instant at which the forward acceleration imparted by the leg thrust is a maximum, and is therefore the instant at which an increased body moment of inertia is most beneficial. The video also showed that as the forward step continues the bird retracts its head at a rate which very nearly offsets the forward velocity of its body (Figures 2 and 3). As a result, the bird's head remains almost stationary during each forward step, thus increasing the acuity of its vision and the efficiency of its feeding.

\midspace{2cm}

\settabs 6 \columns \+ \hfill Figure 1: \hfill & \hfill & \hfill Figure 2: \hfill & \hfill & \hfill Figure 3: \hfill &\cr \+ \hfill Head thrust forward \hfill & \hfill & \hfill Step proceeds; \hfill & \hfill & \hfill Head fully retracted: \hfill &\cr \+ \hfill suddenly; step begins \hfill & \hfill & \hfill head retracting \hfill & \hfill & \hfill step completed. \hfill &\cr \+ &&&& \hfill (Now return to Fig. 1.) \hfill &\cr

The retraction of the head may make another small contribution to body stability by imposing on the body a small angular acceleration about a horizontal axis (which by Newton's Third Law would be in an anticlockwise direction in Figure 2). This would help to offset the angular acceleration of the body in the opposite direction which results from the forward force of the propelling leg. This effect appears, however, to be smaller than those noted earlier.

Evidence which offers strong support for the foregoing theory is the absence of head oscillation in species which are traditionally not ground-feeders. Seagulls, ducks and swans are examples of species which normally feed on fish or weeds, in a watery environment where walking plays no part in the feeding process. When such species do feed on land they do exhibit head oscillations because they have not been bred to do so. If seagulls maintain their modern habit of land-feeding on scraps provided by the human race, and if they abandon their marine-feeding, then after many millions of generations they will begin to exhibit head oscillations when walking.

Examples of species with marked head oscillation when walking are pigeons, doves, magpies, mudlarks, mynahs, starlings, ravens and coots. If a bird oscillates its head, it is a ground-feeder. The converse is not always true; sparrows (always) and blackbirds (usually) hop with both legs together when feeding, and this symmetrical leg action induces no body twist; consequently there is no advantage in head oscillation, and it is not practised by these ground-feeders.

It is interesting that such tiny influences as slight reduction in waddle action and smaller head velocity in mid-stride can, after countless ages, result in every member of a species possessing the same head oscillating characteristic. What makes this occur is not the magnitude of the influence but the enormous time over which it extends.

In summary, some birds are dynamicists in the sense that they make use of The Principle of Angular Momentum to improve their ground-feeding. Like many students they are slow learners, a great many generations passing before the principle is absorbed enough to evidence itself by way of head oscillations.