Birds evolving to fight alien parasites in Galapagos

The parasitic pox virus created the large lesion next to the bill of this medium ground finch. University of Utah biologists are trying to determine if the finches can withstand invading parasite species like the pox virus and the nest fly.

Darwin's Finches Develop Antibodies to Flies & Pox Virus

January 2010. Unlike Hawaii and other island groups, no native bird has gone extinct in the Galapagos Islands, although some are in danger. But University of Utah biologists found that finches - the birds Darwin studied - have developed antibodies against two parasites that recently moved to the Galapagos, suggesting that the birds can fight the alien invaders.

Helping or hurting the birds?
With the discovery that the medium ground finches produce antibodies aimed specifically at the parasites - a pox virus and a nest fly - "the next step is to determine if this immune response is helping the birds or hurting the birds," says University of Utah biology Professor Dale Clayton, who led the new study.

One cannot assume the immune response will help because antibodies also can be involved in autoimmune diseases and allergy symptoms.

Finches are evolving to cope with new problems
Nevertheless the new study is significant because "these finches are icons of evolution, and the icons are in danger of extinction," Clayton says. "Are they sitting ducks? Are they sitting finches? To answer that, the first question is, does the immune system recognize the parasites? And this study shows, yes it does."

Clayton says a key finding is that "wild species can respond to invasive parasites with which they have no history of association. The immune system has been activated."

A researcher holds a baby finch whose nares or nostrils have been chewed and destroyed by parasitic nest fly larvae. The nestling's ears also may have been chewed by the larvae. University of Utah researchers determined the birds develop antibodies specifically aimed at the parasites, but more research is needed to learn if the immune response helps or hurts the birds. Credit Sarah Huber, University of Utah.

A tale of two parasites
The study involved two parasites that invaded the Galapagos Islands. One is the pox virus, Poxvirus avium. Koop says it "creates lesions on non-feathered parts of a bird - around the bill, eyes, legs and feet. Toes and feet can fall off."

The study's main focus was the other parasite, the nest fly, Philornis downsi, which was introduced to the Galapagos as early as 1964. The fly larvae infest finch nests and attack featherless skin, impairing the growth of nestling birds and even killing them.

"The flies can create open sores on nestlings, and decrease survival," Koop says.

15 species of finch on Galapagos
There are 15 species of finches in the Galapagos. All evolved from a common ancestor. Darwin observed changes in their beaks and other features over time, so "they figured prominently in his thinking about how new species evolve," Clayton says.

The Galapagos is "the most famous group of islands that hasn't had any native birds go extinct yet," Clayton says. "Many of the native species in Hawaii, for example, have gone extinct because of humans," who introduced mosquitoes with malaria as well as predators such as cats and rats, and destroyed habitat and hunted birds for feathers.

No native birds have yet gone extinct in the Galapagos because "there were few people living there until the mid-1800s," he adds. That may change with the introduction of nest flies from elsewhere in South America and of mosquitoes that carry the pox virus.

The researchers collected ground finches in 2008 at two Galapagos Islands about 5 miles apart: Isla Daphne Major and at El Garrapatero on Isla Santa Cruz. Nest flies were found on both islands, but pox virus was found only on Daphne Major.

On Daphne Major 30 finches were captured and it was noted whether the birds had pox sores or signs of prior pox infection, like scarring or lost toes.

Larvae of the parasitic nest fly are shown in this photo of a finch nest from the Galapagos Islands. Nest fly larvae attack and can kill the nestlings of medium ground finches, birds made famous in evolutionary studies by Charles Darwin. A University of Utah study found that the finches' immune systems are activated by exposure to the nest fly and to another parasite, the pox virus, raising hope the birds can defeat the alien invaders and survive. Credit Andrew Hendry, McGill University.

On Santa Cruz, finches were examined before and during nesting, which is when the birds are exposed to fly larvae that infest their nests. They captured 37 birds exposed to nest fly larvae, and 76 that were not. They found that 96 percent of nests were infested.

The study found that:

• Finches on Daphne Major had an antibody response to pox virus three times stronger than the response by Santa Cruz finches, which showed no signs of the virus.
• Finches on Santa Cruz that were tested during nesting had an antibody response to nest flies 1.7 times stronger than the response by birds tested before nesting.

Host vs. Parasite: An Evolutionary Arms Race

When parasites invade a "naïve" population, "the question is can the host evolve defences fast enough?" Clayton says. "It's what we call in evolutionary biology an arms race between the host and the parasite."

It is unlikely that the finches had already evolved defences to the pox virus and nest fly, but for yet-unknown reasons, they "have genetic diversity that lets them mount immune responses to parasites, including ones they haven't seen before," he adds.

Immune response - Good or bad?
Koop now is studying whether the finches' immune response helps them or makes them feel ill and less likely to mate, feed chicks, watch for predators and defend territory.

Clayton says such research is urgently needed "because in theory these flies could lead to rapid extinction of bird species" - and not just finches - in the Galapagos.

"Species have long histories of evolving together," says Clayton. "This can lead to a balance. The parasites use hosts but don't drive them extinct because the hosts fight back. But if you pick up a parasite from one spot on Earth and drop it on another spot - something people are doing frequently - then the host animal may not have a chance. There are lots of invasive parasites. This is a big problem worldwide."

The study was published online Wednesday, Jan 6. in PLoS ONE, a journal of the Public Library of Science.
The new study was funded primarily by the National Science Foundation.