Birds are known for the songs they can sing, but dozens of species also use their feathers to generate sounds. In the new study, researchers investigated fork-tailed flycatchers -- 1-ounce birds found throughout the Americas that resemble black-and-gray swallows. The researchers also set up a taxidermy hawk in a field with a hidden camera, and when the fork-tailed flycatchers swooped in to attack, the researchers recorded how the flycatchers’ feathers moved and what sounds they made. The researchers suggested a group of migratory fork-tailed flycatchers ceased to be migratory, and as their wing feathers thickened because they no longer made long journeys, they ended up sounding different from those of their migratory relatives. The scientists will also explore whether female fork-tailed flycatchers prefer sounds from males of their subspecies, Gómez-Bahamón said.