Saving Darwin’s finches from blood-sucking parasites

Saving Darwin’s finches from blood-sucking parasites

In two small areas of mangrove forest on the west side of Isabela, the largest of the Galápagos Islands, the 80 or so remaining members of one of the world’s most endangered bird species are in serious trouble. Philornis downsi, a parasitic fly brought to the archipelago by humans, lays its eggs in the nests of mangrove finches. The larvae crawl out at night and suck blood from the nose and ear cavities of chicks, resulting in malformations that may be fatal or leave them too weak to fledge.

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Tree finch chicks that died as a result of Philornis downsi infestation on Santa Cruz Island. Photograph: Sam Wong

Philornis infestation has affected many species of Darwin’s finches – denizens of biology textbooks and flag-bearers for evolutionary theory – but none is more threatened than the mangrove finch.

Scientists charged with protecting the islands’ birds decided to take drastic action. After camping for 10 days on the beach, surveying the forest for nests and rigging ropes in the trees, project leader Francesca Cunninghame climbed 10-15 metres into the canopy and began lowering nests to the ground in a basket. A total of 21 eggs and three chicks were placed in a portable incubator powered by a car battery and transported 80 miles by boat to the Charles Darwin Research Station on Santa Cruz island.

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Tree finch chicks killed by blood-sucking Philornis larvae. Photograph: Sam Wong

They will be raised in captivity before being released back into the wild once they are big enough to fend off the parasites.

At the research station, Birgit Fessl took me through an airlock vestibule to the small lab where the captive chicks were being looked after, fed on a diet of egg, papaya and mealworm. Eight tiny but healthy-looking chicks chirped incessantly while Fessl told me about the joint conservation project by the research station, the Galápagos national park service and San Diego zoo.

Fessl and her colleague Sabine Tebbich first discovered Philornis in woodpecker finch nests in the Galápagos in 1997. They soon found it in every other bird species they examined. Alongside other factors such as habitat destruction and predation by other invasive species such as rats, the parasite has contributed to declining numbers in several native birds, and perhaps the disappearance of some species from certain islands.

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Mangrove finch chicks that hatched in captivity at the Charles Darwin Research Station in the Galápagos. Photograph: Sam Wong

There has not been a bird extinction in the Galápagos since before Darwin’s time, but without the team’s intervention, the mangrove finches would likely be the first to go. The captive rearing plan is part of a project that began in 2006 to learn more about the mangrove finch and how to protect it.

The project’s funding will end next year, and UK charity the Galápagos Conservation Trust has begun an appeal to support its continuation.

Soon after my visit, the eight chicks joined seven moved the week before to pre-release aviaries specially built in the mangroves. Two weeks ago, all 15 were released into the wild, fitted with transmitters so the team can continue to monitor them closely.

“We will see how many survive the first year – that will be crucial,” Fessl said. “If 10 survive, that would be a really good result. We hope it will be a short-term measure to boost the population. In the long term we don’t want to have caged birds here.”

Earlier this month, a study by Sarah Knutie from the University of Utah tested an ingenious way to get finches to fumigate their own nests. They set up dispensers for cotton laced with insecticide, which the birds incorporated into their nests, protecting them from infestation.

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Several species of Darwin’s finch used insecticide-laced cotton to line their nests in a study published earlier this month. Photograph: Sarah Knutie/University of Utah

Subject to a risk analysis and approval from the Galápagos national park directorate, this strategy could help protect birds in the near future, but other measures will be needed to keep Philornis at bay in the long term. Could the fly be controlled with another parasite? If one could be found that is sufficiently specialised to this host, so that it won’t affect the islands’ native insects, it could be a promising strategy.

Wasps called parasitoids lay their eggs inside other parasites, and tend to be very specialised. George Heimpel from the University of Minnesota, an expert on these organisms, heard about Philornis a few years ago and decided to look for a wasp that attacks it. “You have to go to the native range and find out what’s going on there, see who the parasite’s natural enemies are,” he said.

Philornis most likely arrived in the Galápagos on a boat from mainland Ecuador, possibly inside fruit or other cargo. But Heimpel, working with Knutie, travelled first to Trinidad and Tobago, where the fly is also native and some studies have been done previously.

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The fly breeding lab at the Charles Darwin Research Station. Photograph: Sam Wong

They discovered that rates of parasitism were much lower in Trinidad – around one in five nests were affected, compared with close to 100% in the Galápagos. The numbers of parasites in each nest were also lower.

“That’s a really good sign,” Heimpel says. “If the levels in the native range are lower, it might be that the reason we have high rates in the introduced range is that it has been released from its enemy. You need this scenario for biological control to work.”

They also found a parasitic wasp that attacks P. downsi, and appears to be new to science. The next step will be to study its interaction with Philornis and its specificity, but they have encountered a minor snag: the flies are refusing to mate in the lab.

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Funding for the project to save the mangrove finch will end next year. Photograph: Sam Wong

“It’s just one of those stupid little things,” he says. “Without that we can’t go forward. You don’t want to know all the little experiments I’m doing to try to get them to mate …”

Once this obstacle has been overcome, Heimpel and his colleagues will go through rigorous tests to find out whether the wasp might harm insects that are native to the islands. He admits that biological control got a “bad rap” from early projects such as the introduction of mongooses to Hawaii to control rats, which was not by scientists but by plantation owners. Regulations governing biological control have tightened since them.

“Now there are a lot more protocols in place to make sure it’s going to be safe,” he said. “There are plenty of cases when we look at it and decide it can’t be done safely, and that very well might be the case here.”

There have been notable successes for the strategy, however. In 2002 – after years of careful study – scientists from the Charles Darwin Research Centre working with the Galápagos national park service released a ladybird beetle to control the cottony cushion scale, a sap-sucking invasive insect that was devastating mangroves and other local flora. The project was a great success, reducing pest numbers by 99% on some native plants and removing it completely from others, with no sign of any harm to non-target species.

Whether or not Heimpel’s wasp can repeat this success, Fessl believes the researchers will come up with a solution to save the mangrove finch and other Galápagos finches from being wiped out by blood-sucking flies. “There are lots of good minds working on Philornis, so I’m optimistic we’ll find something.”

This article was written by Sam Wong for The Guardian.

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