Parasites Are Ruining the Love Songs of Darwin’s Finches

The larvae of an invasive fly warp the birds’ nostrils, which in turn impacts their ability to stay in tune

Fledgling tree finches may be infested in the nest. Dr Katharina Peters, Flinders University

The finches of the Galapagos islands unwittingly played a pivotal role in scientific history, helping Charles Darwin formulate his theories of natural selection. Today, Darwin’s finches are in trouble, threatened by a parasitic fly that feeds on the blood of hatchlings, sometimes causing entire nests to die. And as Ian Sample reports for the Guardian, a new study shows that even finches that survive the parasite suffer a worrying long-term effect: damage to the nostrils that warps the birds’ love songs, making it difficult for them to find mates.

Philornis downsi is an invasive fly that was introduced to the Galapagos accidentally in the 1960s and has been wreaking havoc on the islands’ land birds. Adult P. downsi lay their eggs in nests, and when the larvae hatch, they move into baby birds’ nostrils, or “nares.” The ravenous larvae then feed on their victims’ keratin, tissue and blood, eventually moving from the nares to feed externally on the developing birds. Often, the hatchlings will die from blood loss. Those that survive are left with nostrils deformed by tissue-chomping parasites.

A team of researchers recently set out to discover how such deformities affect the song of Darwin’s finches, the collective name for approximately 13 distinct species in the Galapagos. Finches’ songs are incredibly important to their life cycle. Males learn the tune—which consists of one syllable, repeated between three and 15 times, depending on the species—from their fathers, and females prefer partners that can hit challenging high notes.

For the new study, published in Proceedings of Royal Society B, the researchers travelled to Floreana Island, where the medium tree finch (Camarhynchus pauper) is critically endangered, largely due to the blood-sucking larval parasites. The team also looked at the small tree finch (Camarhynchus parvulus), which is not considered threatened, and a hybrid of the two species.

“Female C. pauper often pair with male C. parvulus, producing hybrid offspring that subsequently pair with C. parvulus and other hybrids,” the study authors explain.

The researchers measured the nostril size of 236 adult male finches, and obtained song recordings of 77 of those birds. Since they weren’t able to observe the birds from birth, the researchers looked at data from 37 babies that had been measured on their sixth day in the nest, in order to “calculate the effect of P. downsi intensity on naris size.” The team also watched their feathered subjects from the onset of the breeding season, when adult male finches build a nest and sing until they are selected by a female. Each nest was monitored until the researchers could determine a breeding outcome. Either the male did not manage to snag a partner over the course of 14 days, or there were signs that he had been successful in his romantic endeavors—signs like mutual preening, a female lining the nest and, of course, egg laying.

Ultimately, the team found that medium tree and small tree finch males with enlarged nostrils produced songs with lower maximum frequencies and higher vocal deviations, which, it seems, was not great for their love lives. Birds with high vocal deviations had to sing for more days before attracting a mate, and their pairing success was “rather low,” according to the study authors. Forty-seven percent of small tree finches and 53 percent of medium tree finches that sang at their nests did not manage to attract mates.

Another important discovery lay in the fact that the song of medium tree finches with enlarged nostrils sounded like the song of small tree finches, which could explain why the two species have been mating.

“When a female medium tree finch inspects male small tree finches in the forest, she pairs with one who produces high quality song, even if that male is from another species,” write study co-authors Katharina J. Peters and Sonia Kleindorfer, both of Flinders University in Australia.

This hybridization may not be an entirely bad thing. The researchers found that the nests of hybrid birds contained fewer P. downsi larvae than those of small and medium tree finches, and the hybrid males had the smallest nostril sizes and highest degree of mating success; only seven percent of them were unable to attract a female over the course of two weeks. It is not clear why the hybrids seem to be less affected by the parasite, but the study authors speculate that perhaps “genetic admixture in the host promotes tolerance of a parasite’s microbiome or confers a genetic benefit to sustain other parasite-mediated effects.”

But all this mating confusion among Galapagos finches could “herald the collapse of species lineages,” Peters and Kleindorfer write. Hybridization, they elaborate, “could potentially produce a new species, phase out one of the species, or cause the collapse of the two existing species into one.” Conservationists are therefore working to bring P. downsi under control in the Galapagos, which will, hopefully, help keep Darwin’s finches in tune.

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