The contrasting white feathers at the sides of Moorhen tails are very obvious, especially when they flick their tails up and down, often fanned. As this one run away from me this morning I wondered what is this flicking for? An aggressive signal to other moorhens had been proposed as a hypothesis. However, observations carried out by Fernando Alvarez suggested a more intriguing reason. When a patrolling Marsh Harrier - a common predator - approached a moorhen, the rate of tail flicking increased as the moorhen dashed towards cover. In addition, the more vigilant the bird, or the further away from cover, the higher the tail-flicking rate was. These and other results suggested that tail flicking is not a signal to other moorhens, but a signal of alertness to the potential predator: 'I can see you!' This might deter the predator from chasing that alert individual and try catch a less vigilant one unawares.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
