Parasites ruin some finches’ songs by chewing through the birds’ beaks

Invasive parasites in the Galápagos Islands may leave some Darwin’s tree finches singing the blues.

The nonnative Philornis downsi fly infests the birds’ nests and lays its eggs there. Fly larvae feast on the chicks’ blood and tissue, producing festering wounds and killing over half of the baby birds. Among survivors, larval damage to the birds’ beaks may mess with the birds’ songs when they’re older, possibly affecting their appeal to potential mates, researchers report June 12 in Proceedings of the Royal Society B.

“What’s heartbreaking, when you’re walking through this beautiful forest, is to hear these medium tree finch males just singing and singing and not being able to attract a mate,” says Sonia Kleindorfer, a behavioral ecologist at Flinders University in Adelaide, Australia and the University of Vienna.

The fly arrived in the Galápagos probably in the 1960s. The researchers studied two finch species on Floreana Island that the fly larvae plague: the critically endangered medium tree finch (Camarhynchus pauper) and the related small tree finch (C. parvulus).

In one life stage, the larvae reside in the birds’ beaks, where they chew up the keratin and soft tissue, enlarging the birds’ nostrils, called nares. Kleindorfer and colleagues wondered how this impacts the birds’ song and the sexual selection that results from it.

So the scientists captured finches, measured their nares and then tagged and released them back into the wild. Then, the researchers recorded and analyzed the songs of 77 birds.

The medium tree finch usually makes a more metallic bell-like sound, while the small tree finches’ lower-pitch tune sounds like “cha cha cha,” Kleindorfer says. But in both species, birds with the most deformed beaks sang at a lower pitch than birds with normal beaks.

“If you have a beak with a gaping hole, you cannot hit the high notes,” she says. For medium tree finches, the deformity meant they sounded similar to a small tree finch with a healthy beak. That may explain why scientists had previously had observed female medium tree finches choosing small male tree finches as partners, instead of males from their own species. The researchers did not observe female small tree finches choosing medium tree finch mates.

The research also suggests that the parasites’ impact on birdsong is affecting the birds’ success in finding a mate. From 2004 to 2014, the researchers tracked the courtships of 52 males, watching the birds during two-week stretches in February that coincided with when males prepare a nest and work to impress a female.

The birds with the most altered songs took 36 to 73 percent more days to woo a female, the team found. And those were the lucky ones. Overall, about half of the seven small and 15 medium tree finches followed never wound up with a mate.

But hybrids — with one parent of each species — fared far better in attracting mates, with song quality having no measured effect on whether these birds made a match. Only 2, or 7 percent, of 30 male hybrids studied remained unpaired. Fewer parasites also infest the nests of hybrid birds, and they tend to have less beak deformation, Kleindorfer says.  

“Because the hybrids have such a large advantage, at least one species, the medium tree finch, will disappear,” predicts Heinz Richner, an evolutionary biologist at the University of Bern in Switzerland who was not involved in the study. In part because of how the parasite messes with their mating signal, the two species may merge into one.