Foraging on the wing: How can ecologically similar birds live together?

UNIVERSITY PARK, Pa. — A spat between birds at your backyard birdfeeder highlights the sometimes fierce competition for resources that animals face in the natural world, but some ecologically similar species appear to coexist peacefully. A classic study in songbirds by Robert MacArthur, one of the founders of modern ecology, suggested that similar wood warblers — insect-eating, colorful forest songbirds — can live in the same trees because they actually occupy slightly different locations in the tree and presumably eat different insects. Now, a new study is using modern techniques to revisit MacArthur’s observations, which are still used as an example in today’s biology textbooks.

Led by researchers at Penn State and the American Bird Conservancy, the new project suggests that how these songbirds coexist is more nuanced than MacArthur originally proposed. The study, which examined foraging behavior, physical characteristics, diet and evolutionary history of warblers, appeared today (April 16) in the journal Biology Letters.

A basic principle in the field of ecology describes how two species that compete for the same basic resources, such as food or habitat, cannot coexist indefinitely in the same area. One of the species will ultimately be more successful and replace the other. In his landmark paper from 1958, MacArthur described how five species of wood warbler that live in the same trees and were thought to be “ecologically equivalent” could actually coexist by foraging in different parts of the same tree. For example, some birds foraged near the trunk and others further out in the branches, while some birds foraged near the top of other tree and others at the bottom.

“MacArthur suggested that we needed to look more closely at species interactions,” said David Toews, Louis Martarano Career Development Professor of Biology in the Penn State Eberly College of Science and an author of the paper. “We wanted to revisit this idea with modern technology, using molecular methods and an evolutionary perspective that weren’t available in MacArthur’s time. This allowed us to not only directly quantify their diet, but also better understand how warblers are partitioning their habitat and how what we see today may have been shaped by competition over the warbler’s evolutionary history.”

The research team studied 13 species of wood warblers that coexist in the forests of northeastern New York, where the birds breed in the summer.  They observed foraging behaviors of birds over 20 years, for example collecting information about the birds’ height in a tree, the density of foliage, and distance from the trunk when eating. They also collected fecal samples over five summers to assess the content of the bird’s diet, which comprises insects and spiders. Additionally, the researchers sourced measurements about each species’ physical characteristics as well as their beaks from two public databases.

“The implication from MacArthur’s study is that, if the birds are foraging in different areas of the tree, then they are eating different things,” Toews said. “MacArthur tried to test this using data from his colleagues, who attempted to analyze fragments of insects from stomach contents, but this was biased toward fragments that weren’t digested, and they were extremely difficult to identify. Now, we can extract DNA from fecal samples and use a technique called fecal meta-barcoding to see which insect species have been eaten by birds.”

The researchers, like MacArthur, found that the foraging behaviors of the birds are strongly different among species.

“A bird’s size and shape are products of many pressures operating over evolutionary timescales, but numerous studies, including our own, show that chief amongst these pressures is foraging behavior,” said Eliot Miller, BirdsPlus Index Manager at the American Bird Conservancy and an author of the paper.

Accordingly, the researchers found that differences in foraging were linked to the birds’ physical characteristics. For example, smaller birds tended to hover more often while foraging, and birds with longer leg bones tended to forage near the ground. Differences in diet, however, were minor.

“The birds we studied collectively consumed thousands of arthropod species, and there was a considerable amount of overlap in their diets,” Toews said. “The differences in diet were quite subtle, and we found that closely related species had more similar diets. This suggests that MacArthur’s emphasis on diet may have been partially misplaced, though the general idea of portioning their resources by foraging in different parts of the trees is sound.”

Toews said that the birds appeared to be opportunistic in their foraging. More than 75% of fecal samples contained the most common insect in the area, a snipefly, and more than half of the samples contained a common, invasive leaf weevil.

“Because of the notable differences in foraging behavior, we believe that competition has shaped how these species forage today, as well as how they evolved to forage over evolutionary time,” Miller said. “But we don’t see many differences in diet, so it’s likely that other forces may have more directly influenced warbler evolution. It is worth noting that these birds migrate to Central and South America over the winter, so competition for resources in those locations as well as the stress of migration may also shape the traits we see today.”

The researchers said they hope to further investigate diet and foraging behavior of warblers in their overwintering locations. They also plan to analyze the nutritional quality of food sources to see if competition impacts this aspect of their diet.  

“MacArthur was right in that these warblers are partitioning their habitat in fine ways, but it’s a little more complex than he originally thought,” Toews said. “People often observe competitive interactions between species at their bird feeders, and studies like this show how these kinds of interactions that happened hundreds or thousands of years ago can shape the species we see today.”

In addition to Toews and Miller, the research team at Penn State includes Andrew Wood, a research technologist in Toews’ lab at the time of the research who is now an environmental genomics researcher at the University of Minnesota, Duluth, and Marcella Baiz, a postdoctoral researcher in Toews’ lab at the time of the research who is now an assistant professor of biological sciences at the University at Buffalo. The team also includes Andreanna Welch at the Durham University in the United Kingdom, Robert Fleischer at the Smithsonian’s National Zoo and Conservation Biology Institute and Adrienne Dale at Texas Tech University.

Funding from the U.S. National Science foundation, Schmidt Sciences, the Cornell Lab of Ornithology, Penn State, the Penn State Eberly College of Science, and the Penn State Huck Institutes of the Life Sciences supported this work.