Finches found only in the Galapagos Islands are under attack from bloodsucking fly larvae, but those that live in cities may have an edge compared with their countryside counterparts: they lose less blood and have higher rates of survival during infestations.
The avian vampire fly (Philornis downsi) was accidentally introduced to the Galapagos archipelago a few decades ago. The flies lay eggs in bird nests, where maggots that hatch later burrow into nestlings’ skin and drink their blood. Infestations of the invasive parasites are often so severe that entire nests die, and they present a dire threat to the islands’ native bird species – including Darwin’s finches, the roughly 18 varied species on the islands that were instrumental to Charles Darwin’s first conception of evolutionary theory.
at the University of Connecticut and her colleagues wondered if the habitats of parasitised birds might affect their odds of survival. Birds’ diets can differ in urban areas due to access to human food waste.
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“Because diet can influence how animals defend themselves against parasites, I thought that maybe an urban diet could help or hurt finch defences against the vampire flies,” says Knutie.
On San CristĂłbal Island, the researchers compared the survival rates of parasitised and parasite-free small ground finch (Geospiza fuliginosa) nestlings in the city of Puerto Baquerizo Moreno with those in a natural area in Galapagos National Park.
Among parasitised birds, 48 per cent of those in urban areas lived long enough to leave the nest. But outside of the city, only 7 per cent of those infested with the flies survived.
Finch faeces samples showed higher chemical signatures associated with eating meat in the urban birds, hinting that they may be scavenging meat scraps, possibly out of trash or at outdoor restaurants in the city. The team’s analysis of blood from parasitised birds in urban areas also revealed more of the iron-bearing protein haemoglobin. A meaty diet full of iron might buffer the loss of red blood cells from the parasites, according to Knutie and her colleagues.
The team analysed the birds’ RNA to compare their gene activity levels, finding that urban and rural nestlings adjusted different immune genes when parasitised. The researchers wonder if exposure to pox viruses – common in the city – have somehow primed finch immune systems against the larvae.
at Flinders University in Australia, who wasn’t involved with the research, notes that differences in diet may influence the strength of the immune response. Parental behaviours may also have an effect.
“Urban parents may not have to travel as far to forage and therefore may be able to feed and attend to the nestlings more often than rural birds,” she says.
at Victoria University of Wellington in New Zealand notes that there is some evidence that treating nests for parasites benefits birds differently depending on their sex. So, she says, it would be interesting to see if male and female finches are reacting equally to Philornis in urban areas.
After determining more precisely why urban finches are doing better against vampire flies, Knutie wants to draw comparisons with nearby islands.
“Other islands have more or less human activity compared to San Cristóbal, so I think it would be interesting to explore the consistency of these [survival] patterns throughout the Galapagos archipelago,” says Knutie.
BioRxiv
