From October 5, 2006
What does the racquet-shaped tail of a turquoise-browed motmot (the bird seen at right) have in common with the tail of a deer and the rhyming gingerbread man of fairy tale fame?
They are all important signals in the communication with predators.
The turquoise-browed motmot has a strange looking tail. The two central tail feathers are elongated and designed with weaker barbs towards the ends of the feathers. These barbs wear away to give the feathers an unmistakable tennis-racket shape. When faced with a predator, the motmot will repeatedly wag its tail from side to side in an exaggerated, pendulum-like way (see video of a related motmot species performing the wag display here). Bold move, you might think – and you would be right. The wag display will often draw the one’s eye to a motmot that might not have been seen otherwise, and no doubt it has the same attention-grabbing effect on predators. So why do it?
A researcher from Cornell University, Troy G. Murphy, recently looked into this problem, studying motmot colonies that nest in abandoned quarries and wells in Mexico. He developed several hypotheses to explain the tail wagging behaviour, and then performed careful observations of the context of over 100 wag displays to discriminate between the explanations.
His hypotheses were as follows: (i) The motmot wags its tail as a warning alarm signal to other motmots, alerting them to the presence of the predator. This means that the signal would be beneficial to nearby individuals (such as kin or mates) even though it might be dangerous for the signaler to draw attention to himself. (ii) The motmot wags its tail as a self-preservation alarm signal. The signal should still be directed to other motmots, but instead of being dangerous for the signaler it might benefit him by encouraging other nearby motmots to move closer together or even mob (attack) the predator. (iii) The motmot wags its tail as a pursuit-deterrent signal directed at the predator itself. Much like the gingerbread man, the tail wag would say, “Run, run as fast as you can, you can’t catch me…” This kind of predator-prey communication would actually benefit both adversaries: the prey gets to stay where he is while the predator avoids wasting energy on what would probably be futile chase.
Murphy found that when presented with a predator, the turquoise-browed motmot will perform the wag display even if it is alone and not within sight of other motmots. He also found that motmots are just as likely to tail-wag when they are alone as when they are near their mate, or near any other motmots. These observations allow us to reject (i) since the display is obviously not intended for motmot receivers. Murphy was also able to reject (ii) since the birds do not move closer together or mob when a predator approaches.
From this evidence we can conclude that the tail wagging must be a signal to the predator, communicating that the motmot has spotted the threat and is ready to escape. Interestingly, the tail of many ungulates has a similar pursuit-deterrence function. For example, some white-tailed deer will signal to chasing predators by flagging their conspicuous tails. Pursuit-deterrence signals have also been observed in lizards (arm-waving to deter predators) and fish (swimming right up and inspecting predators directly to deter them). Too bad for the gingerbread man – if he had stayed in one place and relied on his signaling he might not have been eaten after all.