Parasites usually impact individual health—they suck your blood or take your food. Sometimes, the really bad ones kill you. In a little island in the Galapagos, a parasite is doing even more: it’s changing evolution.

According to a new study from researchers at Flinders University in Australia, a parasite is attacking the beaks of two finches—the small tree finch ( C. parvulus) and the critically endangered medium tree finch ( C. pauper). Darwin made these finches famous on his voyage to the Galapagos, as they helped him come up with his theory of evolution. Now, a parasitic larvae is making them evolve in new ways. It’s changing their mating song, confusing female finches so much that they sometimes mate with the wrong species of finch. A new species is being created: a hybrid of those two birds.

“The introduced P. downsi fly has been wreaking havoc on the survival of Galapagos land birds since its larvae were first discovered in a Darwin’s finch nest in 1997,” the study said. “Identified as the biggest threat to the survival of all Galapagos land birds, the larvae of P. downsi kill more than half the nestling finches and leave the remaining surviving birds with various degrees of [nostril] malformation.”

The culprit is the larvae of Philornis downsi, a tiny fly introduced to the Galapagos in the 1960s. The fly larvae hides inside the beaks of the finches, eating the blood and tissue from the inside out. These flies can completely change the beak shape, nearly doubling nostril size. In severe cases, the beak will end up open on both sides, meaning its whole nasal cavity is missing.

If you’re one of these finches, this nose change is more than just a beauty problem. The finches sing to attract mates, and any change to their beaks has serious implications for their songs. In their paper, which was published in Proceedings of the Royal Society B, scientists said that birds with beaks deformed by parasites have more deviation in their songs. Normally, these two finches have totally unique calls, which helps to announce their eligibility to potential mates. When these males change their songs, it confuses females.

Instead of mating with males of their species, females are increasingly mating with males of the other finch species. This has led to a larger population of hybrids, whose songs are now indistinguishable from the calls of small tree finch. With an unaffected beak, medium tree finch males produce their own, completely unique song. When the parasite deforms their beak, though, medium tree finch starts sounding a lot like the other two finches.