As male fork-tailed flycatchers zip around, their wings can produce a high-pitched trilling, and new research shows these whistles have dialects and may be used for communication.

Fork-tailed flycatchers (Tyrannus savana) have two subspecies: one that migrates annually between northern and southern South America, and another that resides year-round in the north of the continent. Valentina Gómez-Bahamón at the Field Museum in the US and her colleagues studied the two populations.

They collected and analysed audio and …