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Colin,
That experiment is an almost ancient and often
cited piece of ethological literature. Reported many times but I wonder how
often it has been independently verified. Also your explanation about the alarm
calls and shape of an egret is also an old interpretation (even if you have
thought of it independently, you are far from the first to suggest that). The
experiment has its detractors though. I recall where someone else reduced the
flying shape to a simple isosceles triangle, lead by either its long end or its
two short ends. The result was the same. So it isn't clear whether the birds
(and it was tested with baby chickens I think, not adult birds that have a
strong predator reaction) respond to the hawk shape or a shape with a sudden
onset- as distinct from a gradual onset. Then other experiments said that
conditioning and frequency of seeing the signal had something to do with it. So
birds get habituated to birds that they see often and don't do them any
harm. Things that are rare and different in shape to the usual, scare them. This
fits in well with the observation that birds may get very defensive or
aggressive to a rare visitor harmless species, like a large pigeon.
Other aspects are that Noisy Miners respond
aggressively to most other birds. They may not perceive a heron as a hawk, they
may perceive it as what it is and yet they want to be alarmed at any bird. I
have seen an alarm reaction of various honeyeaters, magpie-larks etc. to a
perched Nankeen Night-Heron, in a situation where occurrence of a Nankeen
Night-Heron would be most unusual (in tree beside building and a city
park).
Philip
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