Finches on the Galápagos Islands have become famous because of the subtle variations in species between islands. These variations represent different selective pressures, showing the birds evolved in response to the environment.
The Galápagos finches, also known as Darwin’s finches, have been scrutinised since the 19th century, but according to a team of researchers from Flinders University, there are still plenty of things to find out.
In the 1960s, an invasive parasite called the avian vampire fly (Philornis downsi) was introduced to the islands. The fly larvae get into the nests of the finches and feed on the blood and tissue of nestling finches, causing – if they’re lucky – deformities in the birds’ nostrils. If they’re unlucky, the finches die.
Baby finches aren’t affected equally by this parasite. Two papers recently published by the team demonstrate that differences in the birds’ genetics and behaviours can make them more (or less) susceptible to the avian vampire fly.
One paper, published in Scientific Reports, found that the fly larvae lived best in the nests of the medium tree finch (Camarhynchus pauper), which lives on Floreana Island. But since the fly’s introduction, medium tree finches had begun interbreeding with small tree finches (Camarhynchus parvulus), forming a hybrid population. The hybrid population was much less susceptible to the parasite.
According to Lauren Common, a PhD candidate at Flinders and lead author on the paper, the hybrid birds are fertile, and look very similar to medium tree finches.
“Generally, they’re very difficult to distinguish in the field, and that’s why we have to ID them by testing them genetically,” she says.
It’s not clear why the hybrid birds are more resistant. “It could be so many different reasons,” says Common. “It could be that possibly the hybrids have a better immune system… it could be the parasite is choosing to not lay their eggs in hybrid nests, because their offspring die at higher rates in the hybrid nests.”
She adds that the parasite is providing an interesting example of evolution in action.
“This tells us a story from both sides; on one hand, it demonstrates that natural selection would favour those flies that target this threatened species, but it also shows us how the finches are fighting back.”
The second study, published in Biological Journal of the Linnean Society, investigated another type of Floreana finch: the small ground finch, or Geospiza fuliginosa.
While handling the birds to measure weight and other health indicators, the researchers noted how much each bird was struggling against them.
“We gently tilted each nestling onto its back and recorded how long it spent struggling over a 30-second period,” says Dr Andrew Katsis, a researcher at Flinders and lead author on the paper.
“We [also] recorded how much each nestling struggled while we took four important body measurements. This behavioural testing only lasted a few minutes for each bird, because we wanted to get them back to their nests as quickly as possible.”
Katsis and colleagues found nestlings that struggled more had received more damage from the fly larvae.
“That actually surprised us, because we thought if you’re a particularly vigorous nestling, then you may be able to better deal with the parasite infestation,” says Katsis.
“But that was actually the opposite of what we found.”
The researchers think the more docile birds could be drawing less attention from parasites because they don’t move around the nest as much – or, possibly, that birds with more parasite infestations behave differently.
Katsis says this finding needs to be followed up with evidence that birds in the hand are equally fractious in the bush.
“That’s a natural next step to look at these behavioural types that we measure in the hand, to see whether that relates to how the birds are actually behaving in the nest.”
If it does hold up, there could be implications for the finch population at large on Floreana.
“We don’t actually know which way the cause and effect goes,” says Katsis, “[but if] we find that there is some selection happening and that the birds that are struggling more are being affected by the parasites worse, then we would predict that over time, there would be a shift towards more docile behavioural traits within the Darwin’s finches.”
Both researchers agree that, despite their scientific popularity, more research needs to be done on Darwin’s finches.
“They’ve been studying them since the 1800s, but there is so much we still don’t know about them,” says Common.