[From Bill Powers (950529.0700 MDT)]
RE: PCT data on behavior
We have a humming-bird feeder hanging on our back deck, with four
hummingbirds who vie for the four feeding ports. This feeder
consists of a bulb and a red ovoid unit below it with four slits
in recessed openings. The assembly hangs on a nylon string about
two feet long.
The hummingbirds zoom up to this feeder at high speed, and stop
dead in the air to look around before moving up to a feeding
port. They change their orientation through 360 degrees, stopping
perhaps three or four times in one scan. What actually stops is
not their bodies, but their heads. The bodies move through an
angle in the vertical plane, but the heads remain level and
stationary when sighted against a background object.
When the birds move up to the feeding ports, the whole feeder is
usually swinging gently in the wind and twisting slightly around
the axis of the supporting string. The head and body of the bird
swing back and forth in space, and the orientation of the bird
moves in an arc around the feeder as it twists, so a constant
distance is maintained between the beak and the feeding port and
the beak is pointed directly into the port. I can't see any
variation in the relationship of beak to feeding port while this
is going on. While the bird is feeding, the angle of the body may
change in a vertical plane, but the head remains fixed in
relationship to the swinging and rotating feeder. The body
appears to rotate vertically about the head.
It takes a while to realize that there is another disturbance
beside the swinging and rotation of the feeder. There is usually
some variable wind blowing, so the bird is actually not
maintaining a position in still air but is flying upwind at
varying relative speeds and directions all the time it is
stationary relative to the ground or the feeder. There is no
visible sign of the effect of the wind or the swinging and
twisting of the feeder on the relationship between the bird's
head and the feeder port.
From the bird's point of view, it is of course the feeder's
distance and orientation that is stabilized. The bird modifies
the beat pattern of its wings (a blur to me) to maintain the
feeder at a constant distance and in a constant orientation
relative to the head. This happens despite disturbances which,
without adjustments by the bird, would have an easily visible
effect on the relationship of bird to feeder. The controlled
variable is probably controlled visually. When the beak is
actually in the feeding slot, tactile sensations may also become
part of the controlled variable.
As seen through my own perceptual systems, the controlled
relationship is not disturbed at all by the movements of the
feeder or by the variable wind speed. Artificial aids would be
required to measure the residual effects of disturbances, to
determine the loop gain and dynamics of the control systems
involved. It would take high-speed cameras to sort out just how
the figure-eight beat of the wings is modified as a means of
varying the relationship between head position and orientation
and the position and orientation of the feeder.
In this case, nature provides disturbances and we can easily see
that they are resisted.
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Best,
Bill P.