More experiments by Alvarez and colleagues suggested that moorhens in better physical condition showed higher flicking rates when foraging: healthier individuals flick their tails faster, which indicates that the signal is actually telling the predator: 'I can see you and I am fast so you can't catch me!
So both prey and predator would benefit from this honest signal of body conditions and alertness, as a chase, which would be wasteful in energy for the prey and likely to be unsuccessful for the predator, is avoided.
flicking away from me
she is definitely seen me!
she looks quite alert, but maybe then realised I pose no danger
UPDATE 3/10/2013
I took this video today of the tail flicking behaviour in the park moorhens
More information
Fernando Alvarez (1993). Alertness signalling in two rail species Animal Behaviour, 46, 1229-1231 DOI: 10.1006/anbe.1993.1315
Fernando Alvarez, Cristina Sánchez, & Santiago Angulo (2006). Relationships between tail-flicking,morphology, and body condition in Moorhens Journal of Field Ornithology, 77, 1-6 DOI: 10.1111/j.1557-9263.2006.00001.x
Thank you for this explanation. I recently saw a pair of Moorhens tail-flicking. A few moments later, a chick appeared, followed by three more. this is on a small pond of water in a river wetland in Cape Town. The adults steered them across a short stretch of open water (about 10 m) until they disappeared between the reeds. I thought perhaps they were signalling to their chicks to follow, but, after reading this, see it was perhaps a powerful message to any would-be predators!
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