How do shorebirds twist and weave in flight as one?
An article in Birding 26(1):56, 1994 explains how shorebirds seem to move
in a flowing motion as a single unit. The article is by Kenneth Able. I
summarized the note in our newsletter
The reason shorebirds do not fly into each other is that the movements of individuals within the flock are highly coordinated. Slow motion analysis of film of wheeling sandpiper flocks reveal that the majority of birds in the camera's field effected a coordinated change in flight orientation in 196 milliseconds (ms) (about 2 tenths of a second). This rapid response is not due to ESP or thought transference, but simply to the ability of flying birds to notice and alter their flight within a very short time period. There is no consistent "leader" in a flock of shorebirds as any observer can see. The whole flock turns repeatedly in different directions so that different birds are "leading", or at least are ahead, each time. Any individual can initiate a flock maneuver, which then spreads throughout the flock in a wave. Flock members appear always to follow the lead of individuals banking toward the flock. This arbitrary rule probably helps to prevent indecision and enables the flock to respond rapidly during attacks by birds of prey.
Once one of these waves has begun, it travels through the flock at a speed that is inexplicable on the basis of what we know about the reaction times of birds. The propagation time of passing the maneuver on from neighbor to neighbor was about 15 ms, nearly three times faster than should be possible if flock members were simply reacting to the actions of adjacent neighbors. It appears to observers as if the birds were all turning almost at once. The rapid response is a type of "chorus line" effect, which means that a bird distant from the initiator of the turn or change in flock direction, can estimate or anticipate the time of arrival of the approaching maneuver to him, and thus respond much faster than the bird adjacent to the initiator. That bird responds in about 67 ms, much slower than subsequent members of the flock that can anticipate the coming maneuver.
Perhaps the seeming indecision of shorebird flocks that sweep back and forth wheeling and turning trying to decide where and when to land is because they are always reacting to the bird turning towards the rest of the flock - which makes the whole flock veer off in a new direction. As soon as all members "agree" where they are heading, they can settle down to a common direction or destination.
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