11-04-2025
Migratory songbird's gut may contain answers
Researchers studying how the Yellow-rumped Warbler — a bird common to Frederick County this time of year — digests wax myrtle berries say the findings could lead to insights about invasive species, ecological impacts and possibly even how to recycle plastics.
Data from eBird, a database of observations compiled by the Cornell Lab of Ornithology, say that observations of the Yellow-rumped Warbler in Frederick County spike both in late April into early May and in October.
The bird appears in over 1 in 3 submitted checklists from the county in the eBird database during those periods.
Robert Fleischer, emeritus research zoologist at the Smithsonian National Zoo and Conservation Biology Institute, said it is "not a rare bird at this point."
Fleischer is an advisor to Alison Fowler, a post-doctoral researcher at the Smithsonian National Zoo and Conservation Biology Institute, on the research into the species that many birders call a "Butterbutt."
The Yellow-rumped Warbler, a migratory songbird, spends its summers in northern Canada and much of the northern United States.
During the winter, the Butterbutt stays in and around Maryland while other types of Warblers migrate further south.
The Yellow-rumped Warbler has evolved to be able to digest wax myrtle berries and bayberries, which are common in the region in places like Assateague Island National Seashore.
Fleischer said that while some robins and cedar waxwing can also eat these types of berries, "in terms of an insectivorous bird, this is very unusual."
This adaptation helps the bird survive winters — at unusually northern latitudes — when insects, a major part of its normal diet, are hard to find.
The waxy berries have high fat content, which Fleischer called "fat bombs."
Exploring the implications of this rare trait inspired Fowler to apply for a biodiversity fellowship with the Smithsonian.
"It provides an interesting insight into these adaptation strategies that we don't see too often," she said.
Fowler has received funding both through a competitive grant and other internal resources, which allowed her to work with a team of over 14 as the project had expanded and grown.
"It's been fun to work with so many types of scientists," Fowler said.
Fleischer said that compared to a research institution, a national network provides for a wider breadth of research collaborators.
"That's the beauty of the Smithsonian National Zoo," he said.
The team caught Yellow-rumped Warblers in nets at Assateague National Seashore and Edgewater, collected fecal samples and released the birds.
They also collected berries to study.
The initial aim was to identify the bacteria in the Butterbutts' gut that allows it to digest the waxy coat surrounding the fatty berries.
That led to questions about how diets change in captivity by studying those Yellow-rumped Warblers at the Smithsonian National Zoo.
The study also looks at the microbiomes of other songbirds, using DNA sequencing, to compare different digestive systems.
They have also looked into ecological impacts.
"She keeps finding new paths to take," Fleischer said of Fowler.
Fowler said that when looking at the whole ecosystem, the researchers wondered if both populations rely on each other.
The wax myrtle grows in such specific regions and creates "monocultures" as Fleischer said, that perhaps the process of digestion helps spread the tree's seeds.
However, the ability to spread seeds may have a downside that the researchers are also looking into.
The Chinese tallow tree was introduced into the U.S. by Ben Franklin in 1776, according to the USDA.
The Chinese tallow is considered invasive by the USDA and has berries that are toxic to cattle.
Fowler said the Yellow-rumped Warbler could inadvertently be spreading the invasive species.
"That's our worry," she said.
Another area of study is how the bacteria in the birds' guts could be similar to those in insects.
Fowler said that insects that can digest wax can also digest plastics because it is similar chemically.
A next step for the researchers will be to test which of the bacteria identified through the research is able to digest wax or plastic on plates.
"It may even help with recycling plastics down the road," Fleischer said.
The researchers said that while they have identified bacteria of interest, they are still collecting data and performing tests.
Fowler said they hoped to submit their findings for publication in around six months.
Stacy Small-Lorenz, landscape and avian ecologist with the University of Maryland Extension, said people interested in seeing the birds this spring will have the best luck looking in forest or green spaces like parks or yards.
