Legacy pollution from coal mining is changing how a native songbird eats, according to a new study published in the open access journal PeerJ by Brian Trevelline, a postdoctoral researcher at the University of Pittsburgh, and Steven Latta, Ph. D., Director of Conservation and Field Research at the National Aviary.
Louisiana waterthrush are small and a drab gray color, and they’re the first migratory birds to arrive in our region from the Caribbean and Central America to breed.
“Trout fisherman may know this species,” says Latta. “The Louisiana waterthrush is sometimes referred to as the feathered trout, because they inhabit similar sorts of streams, and even forage on similar sorts of insects.”
Building on research collected at Powdermill Nature Reserve since the 1990s, Trevelline and Latta found that the acid mine drainage from abandoned coal mines, which affects thousands of miles of Pennsylvania streams, is changing the diet of the Louisiana waterthrush that breed on those streams. The Allegheny Front’s Kara Holsopple recently sat down with the researchers to learn more.
Kara Holsopple: How does acid mine drainage impact water and the diet of Louisiana water thrush?
Steven Latta: The acid mine drainage increases the acidity of the water, which in turn impacts the macroinvertebrates, the small stream insects that the Louisiana waterthrush depends on.
KH: What kind of insects are we talking about?
Brian Trevelline: Ephemeroptera, which are mayflies, Plecoptera, that’s stoneflies, and Trichoptera, or caddisflies. Fly fishermen probably know these insects pretty well, and they’re ordinarily sensitive to all types of organic pollutants, as well as acidification. It just so happens that waterthrush are particularly reliant on pollution-intolerant insects. They tend to gravitate towards those for their diet, and for the diet of their young.
SL: We’ve shown that when these insects are not present, the Louisiana waterthrush nest at much lower numbers, so a much lower density of birds nesting on these streams. They lay fewer eggs, and the chicks that hatched from these eggs are smaller. That suggests that they’re going to have much lower survival rates.
KH: So how do you know what Louisiana waterthrush are eating to know if their diet has changed? This is the part that I found the coolest in your study.
BT: So studying the diets of songbirds is actually pretty difficult. In the past, researchers have used chemicals like ipecac or stomach pumping to force birds to regurgitate their stomach contents. Obviously, this is pretty stressful for those birds. You wouldn’t necessarily get great data from that either, because you have to identify insects based on semi-digested little pieces of wings and legs. So what we wanted to do was analyze waterthrush diet using a non-invasive technique just using the fecal samples.
KH: So you look at the DNA of the insects that they ate?
BT: Right. The technique itself is called DNA metabarcoding, and it’s actually pretty straightforward. Plants and animals all have DNA. And so the sequence of bases within the DNA, meaning the A, T, C and Gs that make up the genetic code, differ between species. These genetic signatures are actually retained even after digestion, so you can identify which insects were in the diet just using the residual DNA present in feces. You can think of this like a crime scene investigation, where you can identify species of insects rather than an individual.
KH: That’s really cool. Is this something that’s new to this kind of research?
BT: I would say that it’s really picked up in the last five years or so. This was something that people were using a lot with bats, around 2010 or so, and our group was one of the first to do it in songbirds.
KH: So if they’re not eating mayflies, and the things that they prefer, what are they eating?
BT: We found that in acidified habitats, they’re eating more moths, crickets and spiders–those types of arthropods that are found further away from the stream, rather than on the stream. This suggests that the birds are traveling further away from their nests in order to get the adequate food for their young.
KH: Is that necessarily negative?
BT: It’s showing that the birds are compensating behaviourally for the reduced availability of their preferred prey, but it could carry other costs that we’re not aware of. That’s what we’re looking at now. Birds go to pretty great lengths to avoid predators, for instance, and the more time they spend away from the nest, the more likely is that a bluejay or a raccoon or a snake will see them leaving the nest or coming back to the nest frequently. So it could affect affect survivability of the young.
SL: Another aspect that we’re looking at is that the quality of these alternative food items may not be the same as the quality of mayflies or stoneflies for waterthrush. There’s some indication that they might not have the calcium that the birds really need in order to lay the same quality eggs. So there’s a lot of different sorts of potential ramifications of diet change. Birds can be really excellent indicators of the health of their environment, and we’ve shown that the Louisiana waterthrush is really an excellent indicator of the health of these riparian systems, these streams systems, especially in the Appalachian Mountains.