In the Galapagos, urban finches fare better against the vampire fly

Scientists are working hard to thwart a blood-sucking fly that is decimating populations of the charismatic finches that helped Charles Darwin formulate the theory of evolution.

In a promising new study led by UConn researchers, they find that finch populations living in urban environments are somehow more likely to survive the harmful impacts of the invasive parasite, giving hope for conservation efforts. The results are published in Biology of global change.

The avian vampire fly, Philornis downsi, native to mainland South America, was first discovered in the Galapagos in the 1950s and recognized as a significant problem in 1997, says Sarah, co-first author of the study and UConn associate professor of ecology and evolutionary biology. Knutie.

Although adult flies feed on decaying organic matter and are not parasitic, they lay their eggs in bird nests where, once hatched, the larvae feed on the blood of nesting birds, often leading to serious consequences. devastating. The fly is now present on almost every island in the archipelago, and scientists are working to determine what the long-term effects will be on native bird populations.

“Almost all studies have found a significant effect of the fly on chick survival and this fly can kill up to 100% of nesting Darwin’s finches in a given year,” says Knutie. “These nestlings die primarily by exsanguination, meaning the larvae suck out all the blood, hence the name avian vampire fly.”

With such high mortality rates, the survival of Darwin’s finches is in question, and figuring out how to control the fly is a major conservation concern, not only for researchers who work with the birds, but also for the Galapagos National Park.

“The fly does not affect all Galapagos bird species equally,” says Knutie. “Some bird species like Galapagos mockingbirds are better defended against flies than Darwin’s finches and we think this is because the mockingbirds compensate for the parasite’s loss of energy through the parents feeding more baby birds when they are parasitized, rather than when they are parasitized. I am not parasitized.”

Knutie began thinking about potential variations in how flies affect Darwin’s finch populations and she also noted that previous studies focused on finches living in natural, non-urban environments, which doesn’t give a complete picture of all finch populations.

“We know that there are different food availability for animals in urban areas, so I began to wonder if urban finches were affected differently or if perhaps they were better defended against flies because they had access to different food,” she says. “These urban areas are different in many ways, not just the availability of food, but that’s where I started because I knew that mockingbirds are better defended because they are able to feed their nestlings more.”

The study took place on the island of San Cristobal, which is home to a large city, Puerto Baquerizo Moreno, with a population of approximately 8,000. The researchers experimentally manipulated the parasite in nests located in urban and non-urban areas, where they removed parasites from some nests. Next, they tracked the finches’ health and survival and collected a blood sample to track blood loss and study gene expression and immune responses.

“We found that when nests were not parasitized, survival of nesting birds did not differ between urban and non-urban areas, so urbanization itself did not affect survival. When nests were parasitized, survival was more than six times greater in urban chicks compared to non-urban chicks, whose survival was almost zero,” says Knutie.

To get to the root of differences in survival between contexts, the researchers took a transcriptomic approach.

Cynthia Webster, co-first author and second-year doctoral student. collaborating student and associate professor Jill Wegrzyn, was charged with understanding the molecular mechanisms behind how parasitism and urbanization influence outcomes for finches, including differential expression of immune genes.

“This transcriptomic approach provides a more holistic view of our system,” says Webster. “It’s helpful to understand the molecular mechanisms behind what we see and that’s where this omics-based analysis adds another level to our study.”

Webster performed gene expression analysis by examining RNA sequence data to see what was actively expressed at the time the samples were taken, specifically comparing urban and non-urban sites, parasitized and non-parasitized individuals. parasitized, and whether the bird died or survived.

“Based on immune pathways alone, we find that much of the differences between urban and non-urban parasitized chicks lie in resistance and tolerance mechanisms. We zoomed in on parasitized urban chicks, where we noticed “The differentially expressed genes fell into pathways associated more with tolerance and innate immune resistance. Meanwhile, non-urban nestlings showed more adaptive immune resistance,” says Webster.

They found that, in general, fledging success in urban birds was associated with a pro-inflammatory response, which they believed could help prevent parasites from feeding. Non-urban birds did express immune pathways, but they believe this could have been a last-ditch effort to try to defend against the fly, but it proved ineffective because most of the birds died anyway. They also found that urban birds likely consumed more protein, confirming dietary differences between urban and non-urban populations.

The results are encouraging, but Knutie wants to be clear: They do not suggest that the Galapagos Islands should be urbanized; however, the results offer hope that there is something about the urban finch population that improves its outcomes.

“If we can determine whether it’s the evolution of defenses or whether there’s an environmental factor helping them, that can help inform conservation or management strategies for other populations, so that Bird populations are not declining or disappearing,” Knutie said. “This gives us hope that the larger populations of Darwin’s finches affected by the fly are not entirely doomed. We are still concerned about smaller populations facing the fly, but perhaps the information of our study can help them.

Knutie says the next step is to determine if urban finches have developed defenses or if there is an environmental factor, such as food availability, that helps finches survive the flies.

“We would also like to expand the study to all islands, because there are four islands in the archipelago with permanent resident human populations and each island has a different sized human population. As you can imagine, as as the size of the city increases, so does the infrastructure to support humans. We hope to test whether the effects of urbanization on this bird/parasite interaction vary from island to island, depending on the degree of urbanization, to see if they are consistent,” says Knutie.

Knutie hopes they can elucidate key mechanisms that will help inform future conservation efforts and contribute to ongoing efforts.

In the natural range, fly populations are likely controlled by natural predators like parasitoid wasps or ants, Knutie says. However, these natural control mechanisms are not present in the Galapagos. Some work is underway to study and explore the potential introduction of natural controls, but these measures can be costly and time-consuming. However, some populations of Darwin’s finches may not have much time left.

“For example, the mangrove finch has fewer than 100 individuals in the entire species, making it one of the most endangered bird species in the world,” says Knutie. “They don’t have time to develop their natural defenses, so scientists and the park have been trying to figure out how to manage this fly for these populations that just don’t have the time. For larger populations, we have a little more time to understand why they don’t do so well against the fly, and maybe help them defend themselves better. If it’s evolution that’s working on urban finches, then maybe “This gives us hope that there is a chance that other populations will develop defenses against the fly.”