When crested pigeons spot a predator, they give a little whistle. But the sound doesn't come from their beaks. And according to a study published by The Royal Society, that whistle warns the rest of the flock to flee. There’s safety in numbers, and a flock offers protection in part because it has so many eyes watching for danger. But it's not clear how an individual sounds the alarm when it spots a predator. Maybe anytime one or two birds fly off, they all do. Just in case. Of course that kind of knee-jerk reaction could produce a lot of false alarms. So how can birds tell when their flockmates are seriously headed for the hills? Scientists recorded the sounds made by the wings of crested pigeons during a routine liftoff. And the noise their wings make during an emergency takeoff . And they found that the sound made by wings flapped in terror includes a distinctive sort of whistle. That noise alone will scatter a flock of feeding pigeons just as quick as the shadow of a passing hawk.