Many unrelated species of birds have a habit of pumping (or wagging) their tails. They are mostly open-country birds like phoebes, pipits, Palm Warbler, Spotted Sandpiper, and others. Many hypotheses have been suggested to explain why the birds do it, but it was only in the last few years that research provided an answer.
In 2011, biologist Gregory Avellis watched Black Phoebes in California and found that phoebes pump their tails regularly all day long, and the rate of pumping does not vary much no matter what the birds are doing. However, playing a recording of a Cooper’s Hawk (suggesting that a dangerous predator was nearby) caused the phoebes to pump their tails three times as much.
The explanation is that tail pumping is a simple signal, and the message it sends to the predator is: “I know you’re there. I’m healthy and quick, and you won’t catch me, so don’t even try.” This matches research on lizards and ground squirrels, which also use tail motions as a predator-deterrent.
This is not to suggest that tail-wagging involves any conscious decision-making by the phoebe. If we could ask a phoebe why it wags its tail all the time, I suspect the answer would be: “Do I? I don’t know, it’s just a habit.”
Tail-wagging can be thought of as a signal that the phoebe and the hawk both understand on a basic level, but it is not a conversation. When we get nervous, we fidget. When phoebes get nervous, they wag their tails, and when a predator sees fidgeting or tail-wagging, it gets the message that this is a healthy and alert animal and probably not worth chasing. That aspect — fidgeting or twitching under stress — is universal, something we “just do” because it is hard-wired and instinctive. What varies by species are the precise movements that send the signal.
Phoebes and a few other species of birds wag their tails, others flick their tails up, others flick their wings, and others bob their heads or call. They’re all different ways of sending the same message.
This article was first published in the September/October 2018 issue of BirdWatching magazine. Subscribe now.