Neck-swaying is a common behaviour in the herons. Watch a Cattle Egret
feeding and it won't be long before you see it. It is interesting that you
mention the head being stationary. The behaviour is to assist in fine
focusing of the binocular vision on the prey and does involve the movement
of the head. Perhaps you spotted the heron after most of the "tuning" was
achieved.
This behaviour is most evident with small prey items such as insects that
are, perhaps, less likely to be alarmed by a gently swaying head getting
closer and closer. I doubt it would work with more sensory attuned prey
such as small mammals. When Cattle Egrets fed on mice, for example, there
was no nonsense - straight in and grab. Mind you, this type of prey would
require less fine tuning with focusing.
While working on egrets I had a hand-raised Cattle Egret that I often
observed hunting. The neck-swaying was extremely exaggerated. This was
probably not surprising for an inexperienced hunter on a steep learning
curve. I have some excellent black and white photos of "Fred" staring
directly down the lens at ground level with the head offset to one side of
the body by at least a full body width.
David Geering
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