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Parasitic fly affects quality of song for Darwin’s finches

Darwin's finches
A fledgling Medium Tree-finch. Photo by Katharina Peters/Flinders University

A parasitic fly that was introduced to the Galápagos Islands in the 1990s has been a big problem for two species of Darwin’s finches for many years. Today a new study suggests the problem is more significant than previously understood.

The fly, Philornis downsi, lays its eggs in bird nests. The eggs hatch into parasitic larvae, and they reside in the nest material and emerge at night to feed both internally and externally on the blood and flesh of developing nestlings. The larvae cause significant mortality for Mangrove Finch and Medium Tree-finch, two of the species that are among Darwin’s finches.

Both species are listed as Critically Endangered on the international Red List. Mangrove Finch, found only on Isabella Island, numbers 40-80 adults, while Medium Tree-finch occurs only on Floreana Island and has a population of 600-1,700. The fly is considered the “most significant threat” to Medium Tree-finch, and it’s also a “significant threat” to the Mangrove Finch.

Now, researchers from Flinders University in South Australia and the University of California-Berkeley say the fly poses another challenge for the birds. They report in a new paper in Proceedings of the Royal Society B that the parasites have damaged the beaks and nostrils (nares) of Medium Tree-finches, causing the birds to sing a sub-par song.

“In our newest research, we show that Darwin’s finch males whose nares have been deformed by the parasite had greater vocal deviation, which females didn’t like during mate choice, and had songs with lower maximum frequency,” says lead author Sonia Kleindorfer. She adds that this also “confused the species identity of the singer.”

The birds with enlarged nares produced songs that are was indistinguishable from songs of other finches. This may be a factor in the ongoing hybridization of Medium Tree-finch with its more common cousin, the Small Tree-finch.

“This research is evidence that parasite-induced morphological deformation can disrupt host mating signal with devastating effects on bird populations,” says co-author Katharina Peters, a postdoctoral fellow at Flinders.

The only bit of hopeful news here is that a few years ago, researchers found they could provide cotton balls soaked in an insecticide for the birds to use in their nests. The insecticide is safe for birds, and it kills the fly larvae in the nests. 

Listen to the difference

Normal song of Medium Tree-finch:


Deviated song of Medium Tree-finch:

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