Introduced parasites that alter their host’s mating signal can change the evolutionary trajectory of a species through sexual selection. Darwin’s Camarhynchus finches are threatened by the introduced fly Philornis downsi that is thought to have accidentally arrived on the Galapagos Islands during the 1960s. The P. downsi larvae feed on the blood and tissue of developing finches causing on average ~55 % in-nest mortality and enlarged naris size in survivors. Here we test if enlarged naris size is associated with song characteristics and vocal deviation in the small tree finch (Camarhynchus parvulus), the critically endangered medium tree finch (C. pauper), and the recently observed hybrid tree finch group (Camarhynchus hybrids). Male C. parvulus and C. pauper with enlarged naris size produced song with lower maximum frequency and greater vocal deviation, but there was no significant association in hybrids. Less vocal deviation predicted faster pairing success in both parental species. Finally, C. pauper males with normal naris size produced species-specific song, but male C. pauper with enlarged naris size had song that was indistinguishable from other tree finches. When parasites disrupt host mating signal, they may also facilitate hybridisation. Here we show how parasite-induced naris enlargement affects vocal quality resulting in blurred species mating signals.