The Inner Lives of Birds

Plenty of birds fly vast distances towards the equator for winter and away again for summer, trying to stay ahead of the changing temperatures of the seasons. And now zoologists have found a single gene that seems to help control where they go. How does it work? The same way a gene inside a person might help them run faster, this one might be giving one group of warblers the stamina to fly further south than their neighbours. Phil Sansom heard from David Toews from Pennsylvania State University...

David - We found this gene that we believe is responsible for differences in migration in a Neotropical migratory wood warbler,

Phil - Really? The genes control it migrating? I would have thought that's all stuff it learns when it's a baby or something.

David - Yeah, these small songbirds, nobody really tells them where to go.

Phil - That's amazing. What are these birds then?

David - So these are Neotropical migratory wood warblers. There's about 110 species, but we were actually focused just on two: golden-winged warblers and blue-winged warblers.

Phil - What do they look like, these ones?

David - They're very colourful; but blue-winged warblers, unlike their name, are actually almost totally yellow with a little black face mask; and golden-winged warblers look very different, they're mostly white and they have a really distinct black face mask and throat. They're genetically very similar.

Phil - And when they migrate, where do they go?

David - These birds live primarily in the Eastern U.S. and Canada, but they migrate to Central and South America.

Phil - How do you figure this out, and track a bird to, I don't know, Central America?

David - That's actually one of the trickiest parts; following a tiny 12 gram bird over its entire annual cycle is logistically very difficult. We can't just put a GPS tracker on one of these birds because the battery that would be needed to power that and transmit data weighs about as much as the bird. And so my collaborators at the University of Toledo use these rather ingenious light loggers; these are little backpacks that you put on a bird where it's breeding, and it just captures passive light throughout the day. And long story short, if you know the precise sunrise, sunset, and midday points, you can get an estimate of latitude and longitude. Now you have to catch the bird the next year to download the data off these backpacks, because again, they don't have the capacity to transmit. So you inevitably... because these birds disperse, some of them don't all survive on their migration. If you are lucky enough or have enough money to put out enough of these backpacks, and get enough of them back, you can actually get individual-level movements for these migratory warblers.

Phil - Okay. So you're learning where the birds go. And then, what, comparing that to the genes?

David - Yeah, there was this really dramatic split between those birds that went all the way to South America to those that went to Central America. And so then we used whole genome resequencing. We found this very strong association with a single region of the avian sex chromosome, and within this single region, we found a single gene; and the gene is VPS13A.

Phil - Oh, that's a mouthful.

David - It is. So this basically is involved in the crosstalk between the nucleus and the mitochondria. This is kind of a gatekeeper protein that allows lipids to move back and forth.

Phil - Why would that be a thing for migration? I don't understand.

David - Yeah. So that is the $65 million question, and we don't know. There are some suggestions that it's involved in regulating these molecular products that are associated with stress, called reactive oxygen species. These are harmful byproducts that are produced when even you and I are exercising. And so the hand-wavey explanation for what different variants of this gene are doing is that it's helping regulate how much of the stressful by-product is being produced. But beyond that, we don't actually have an obvious clue of what this gene is doing.

Phil - If you're right: if you have one version of this gene, you're flying, you got a bit stressed from it, you drop down, you're in Central America; but you have the other version, you don't get as stressed, you're fine to keep flying for a bit, all the way down to South America.

David - Yeah, that's the idea.

The coronavirus seems likely to have come to us from bats, possibly via a third party like a pangolin. If so, it would be far from the first time that an animal virus decided humans looked similar enough to make the jump - think of bird flu, mad cow disease, or even HIV. Zoologists from the University of South Florida have therefore been looking into the ways animals live closely alongside humans, and in particular testing out how light pollution affects sparrows that are carrying a common but dangerous disease called West Nile Virus. Phil Sansom heard from Meredith Kernbach...

Meredith - House sparrows, when they're exposed to light at night, they're not able to cope with infection with West Nile virus as well. They maintain the disease for longer and may actually die more easily from it.

Phil - So the light seemed to screw with their health in some way? And they couldn't deal with the virus as well?

Meredith - Right. Correct. So individuals are actually staying sick for longer, which may increase their opportunity to cause a West Nile virus outbreak in their area. West Nile virus is one of those things that spills over to people in the United States, and in Europe, and in Africa. And basically, we're broadly interested in why these diseases spill over. And so we chose light at night because we also know that light at night is rapidly expanding across all continents, except for Antarctica of course; but we know that it has a lot of other negative effects.

Phil - What was your version then, of a sparrow in a suburb? Presumably you didn't actually release sparrows with viruses into the suburbs of a nice city.

Meredith - Right. So we performed experimental infections in captivity, which has its caveats, because it's removed from nature. And these sparrows are housed basically on one side of the room, and we rig up a small light bulb, like a light bulb you'd find in your house, on the other side of the room, at a dim enough level that you might find in nature.

Phil - And then you give them a bit of the virus?

Meredith - Right. So we expose them to a very low amount of virus.

Phil - What does that look like - a sparrow with West Nile virus?

Meredith - It depends on the sparrow. There's a lot of different variation. Some birds will fluff up or look lethargic, and then other birds you'll never even know that they were sick.

Phil - If it's hard to tell whether these birds are actually that sick sometimes, what are you measuring to figure out how bad the light pollution is hitting them?

Meredith - We'll measure an individual's body mass, which indicates how well they're coping with disease. You might assume that a bird that's losing a lot of body mass is a very sick bird. And then we'll also take a blood sample to measure how much virus is in circulation, or how sick they are by how much virus they have in their blood sample.

Phil - And what do you find? How big a deal is light pollution for that?

Meredith - Light pollution actually does end up being a pretty big deal for these sparrows. The sparrows that are exposed to light at night actually have much higher amounts of virus in their blood. The hormone melatonin might be involved. It's a hormone that's secreted at night, but is suppressed in birds that are exposed to light at night. And melatonin is a regulator of the immune system, which means, without melatonin, your immune system might be all out of whack. Another mechanism is that the immune system will fluctuate throughout the day. So certain immune cells may be at higher concentrations, or there may be more of one immune cell during, say, the daytime. And so when they don't have the difference between light and dark anymore, their immune system may just get all scrambled up with not knowing what time of day it is.

Phil - Now, we're on Naked Genetics here. Do you have any clues for me from the genes of these sparrows?

Meredith - Yeah, absolutely. So, in another study we performed a transcriptomic analysis, which basically tells us which genes the sparrows are upregulating in order to cope with West Nile virus infection. And the birds who are exposed to light at night, upregulate these genes that are associated with say, pathogen resistance; which is counter-intuitive because you would expect that the birds with more virus would have worse resistance, but these birds that have more virus are actually upregulating these pathways earlier. And we also found from some of these other genetic signatures that individuals that are exposed to light at night are incurring more pathogen-induced damage, which means suffering a lot more from infection.

Phil - Huh. So it seems like whatever light is doing to these birds, the genes are desperately scrambling to keep up.

Meredith - Exactly.