In his writings on sexual selection, Darwin devoted equal space to non-vocal and vocal communication in birds. Yet, in the years that have passed since his death, the research community has given almost no attention to non-vocal communications. This, particularly when compared to the extensive attention that bird calls have received, is a great shame. Darwin called the sounds that birds made with their feathers and feet "instrumental music" and speculated that they used these sounds to send valuable signals to one another. That hypothesis is very old and has never been tested. Now, a team is revealing that one species of pigeon creates a unique warning sound with a single wing feather when fleeing danger.
The new work was all conducted through a clever mix of video and feather-removal experiments that were able to demonstrate that a highly modified primary feather on the wings of the crested pigeon produces a distinct note during each flap of the wing. Crucially, the note changes with wing beat frequency which means that pigeons fleeing danger produce a sound that is different from pigeons that are merely taking off in a relaxed manner. You can read more in The Economist article that I wrote on this here.